Tilting Trains

Swedish X2000 tilting train with a top speed of 125mph. You can clearly see it tilting here on a test track.
Originally written by Oliver Keating & Lexcie, revamped by Parthiv. 

Problems with corners

We all know that if your driving in your car and you take a corner at speed you feel centrifugal forces. Well it is no different from trains, if a train takes a corner at speed then centrifugal forces come in. Often train operating companies face a decision for building a high speed railway transport system... they can either invest money in the train to make it tilt but use existing railway lines, or they invest money in a new railway but don?t need to spend money on expensive tilting mechanisms. This is why TGV, and ICE and bullet trains do not tilt, because they have their own dedicated high speed railway lines where curves are built with very high radii.

It is worth pointing out that the centrifugal force is a function of v²/r where v is the velocity and r is the radius. This means if you double the velocity, you quadruple the centrifugal force. Similarly, if you want to triple the velocity but keep the centrifugal force the same, you must increase the radius by a factor of nine! Something not always possible. This is why even apparently gentle curves can be much more of a problem with high speeds than one might thing, because the force rises with the square of velocity.

So what's the deal with passenger comfort?

One might think it is not safe to push a train round a corner at high speed. Indeed that is true. However the crucial thing is that the speed at which it becomes unsafe, i.e. the speed at which there is enough force to push the train off the track is incredibly high. In fact the force needed would be enough so that all passengers on the train couldn?t move and would be stuck to the sides of the train. This implies that there is scope for increasing the speed of a train round corners a great deal without it becoming unsafe, however passengers will object.

There are two reasons why it is bad for passengers. I have been on a late running InterCity 125 and it took a corner at some speed although only about 90mph and the corner wasn?t tight enough to say see the train at the other end by looking out of the window. The corner wasn?t tight enough to notice turning either. However you could feel the forces definitely, and empty cups rolled across tables, bags creaked and I felt pushed against the wall.

Any faster and it would be extremely uncomfortable for passengers and they would rather the train slowed down and take a little longer. Also companies don?t like to run trains at speed round corners because it upsets passengers, as when exposed to turning forces they may become worried the train is going to fly off the tracks (which incidentally has never happened*). This might make people nervous about travelling on high speed trains. The reason no one is ever nervous about travelling at nearly 200mph on a train is because it is smooth and constant. If it was doing corners at these speeds people wouldn?t like it. People get freaked out doing 50mph on a roller coaster which involves tight curves, imagine how they feel when serious speed is involved! Its all down to the forces a passenger feels, if a passenger feels a lot of strong forces then he or she is going to be nervous, and may avoid travelling on the train, or just choose a slow train.

* Yes it has: Morpeth 1969, for example, when the up Scotch sleeper express flew off the tracks after it tried to take a 20mph curve at unchecked speed of around 80mph. The haulage was a class 47 which stayed on the tracks, but the lighter coaches were scattered everywhere. The cause was that the driver fell asleep. However, such things don?t happen often; trains regularly pass through corners at upwards of 20% more than the specified speed limit and gets away with it. -Ed.

In order to attract customers, a high speed train has to be fast without feeling like a roller coaster.

Well a railway isn?t all curved, why should the odd curve matter?

The British Advanced Passenger Train, note that the first and the fourth carriages have failed to tilt. The carriages returns to an upright position in the event of a tilting mechanism failure.

You might think that the odd curve in the railway shouldn't be so much of a big deal, the train just has to slow down for the curve and speed up again. Well think of a typical curve on mainline railways. Curves are not particularly tight so it might give a maximum linespeed of 100mph which is fairly common. 100mph doesn't sound slow, no problem. However take a 186mph train. The deceleration of the train is limited by law to 5ms-2, but usually less for passenger comfort. Now after the curve the acceleration is about 1ms-2 meaning it would take a couple of minutes to get back up to 186mph. But in 2 minutes at these sorts of speeds it would have travelled about 5 miles. Now if there is a 100mph curve every few miles... the high speed train is rarely going to get up to 186mph and even if it does it will be for very short runs. Now if it could achieve say 140mph (40% more speed) around curves it would increase the average speed by a large amount, as not only would it go fast around the corners but because less acceleration/deceleration is needed it can travel at 186mph for a much longer period of time. This is why it is extremely desirable to get rid of the curve problems.

Why Titling helps

When sitting on a corner going at speed there are two forces acting on you, gravitational force and the centrifugal force which is accelerating you into the corner. In physics when two forces act, then this causes a resultant force. The resultant force will push you into your seat and to the side. However if the train is tilting, then the normal contact force of you on your seat will be the same as the resultant force you are experiencing. This means as far as the passenger is concerned he or she is just being pulled into his or her seat, and he or she is used to that so no discomfort is felt.

This is true also of aeroplanes, commercial planes tilt a large amount, up to 30 degrees when going around corners in some cases to cater for passenger comfort. As the tilting of the aeroplane is to get rid of the problem of centrifugal forces, or more accurately to disguised the centrifugal forces as a part of gravity as far as the passengers are concerned. The only way you know if the aeroplane is tilting is to look out of the window.

Trains that tilt can go up to 25% to 40% faster around curves than conventional trains without upsetting the passengers, and as mentioned before this can significantly increase average speeds and cut journey times.

Why not just tilt the track?

Well on high speed lines the track in the UK is heavily banked up on corners, although going in a high speed train you don't notice it at all. Occasionally when a high speed train in the UK comes to a stop because of a red signal or something on a curve you can really notice how much its slanted, on a stop on a curve put a bottle on the floor and will slide across to the other side.

However there are limitations with tilting tracks, first of all the banking has to be designed with a specific speed in mind. A banked up track meant for 125mph trains is going to cause discomfort to passengers in a local 75mph train, as when a slower train goes round a banked corner it will make passengers feel like they are falling to a side. Of course you could build dedicated high speed lines, but then you would engineer them without tight curves. This limits the extent to which tracks can be banked up. If the track is banked too much for really fast trains, then if any train comes to a stop on the curve due to a red signal the slant will cause discomfort to passengers. Also arranging for the overhead pantrograph to make proper contact with a wire above a banked curve is a little tricky.

Clearly trains themselves need to tilt, then you get the double benefit of tilted track and tilting train, and the train can tilt to exactly suit the speed it is going at.

How do trains tilt?

Carriages have tilting mechanisms. Obviously the bogies cannot tilt because they ride on the track and must follow the path of the track. So the coaches have to tilt on the bogies. The way they do this is simple, the bogie acts a fulcrum in the centre and it is free to tilt either side. Then pistons control how much the coach tilts. The pistons are controlled by a small computer, which uses a spirit level. The spirit level is used to check if things are horizontal remember, i.e. at right angles to the resultant force acting. Normally this force is gravity, but when going round a corner the resultant is a combination of gravity and centrifugal forces. This means the spirit level indicates it is no longer horizontal, so the computer adjusts the pistons until horizontal is read. Again this will not be horizontal to the ground, but as far as anyone on the train is concerned it will be horizontal, keeping the passengers happy.

In the early days they tried to use inertial force to let the trains tilt? ie they would have no mechanism to make them tilt but the carriages would have a low centre of gravity so centrifugal forces on the carriage would cause them to tilt. This proved unsuccesful.

Countries who developed tilting technology

Britian. The UK was interested in developing the Advanced Passenger Train for quite a while.

France. The development of the tilting train in france began in 1956 when eng. Mauzin built and experimented a single car unit that used inertial (non-assisted) tilting. This experiment were suspended because a natural tilting proved too difficult to accomplish. France preferred to built a vast network of high speed lines and the development of TGV started in the early seventies with a two-car modified turbotrain. Today GEC-Alsthom has decided to start the development of a tilting TGV that has been delivered few weeks ago for testing and trials.

Spain. With the ETR-401 Fiat delivered in 1976 a wide-gauge version to Spain that was designated Tren Basculante (RENFE Type 443). However like its italian sibling it remained a single vehicle. Later Spain developed a tiliting version of its own Talgo train (talgo pendular) that so far has proved itself the only succesful example of natural tilting and has met with a huge success.

USA/Canada. The first experiments were carried over in the thirties vith non-powered cars (called pendulum), but the first succesful tilting train in the USA was the Sikorsky TurboTrain which incorporated an inertial tilting mechanism. Later, in 1973 Amtrak tested an active tilting train called LRC (Light - Rapid - Comfortable) made of ten cars, that unfortunately had no success. Today Amtrak is trying again with the American Flyer built by Bombardier using some TGV-derived technologies.

Sweden. ABB developed in the early seventies an active tilting mechanism that was alternative to the Italian and British ones. This was tested in the X15 vehicle and implemented in the X2000 series train that so far has had a good success. One curious feature of this trains is that the tilting mechanism is applied only to the passengers cars and not to the driving motor units.

Switzerland. As a part of the IC-2000 project the Swiss railways are developing a tilting train that has the provisional designation ICN-2000 and will be built by SIG.

Germany. Germany has adopted tilting technology on its 610 and 611 class series EMU and will built the ICT for long-range intercity services.

Summary

While the very high speed trains like the TGV could be regarded as the Rolls Royce of trains, tilting trains could be thought of as the cheep and cheerful mini metro. The price differential is fairly similar too, it costs about 20 times more per unit distance to build a dedicated high speed line than it does to upgrade existing lines for tilting trains. This is what makes tilting trains extremely attractive. However there are disadvantages. 140mph is about as fast as trains go when not on dedicated lines. And then they have to be fitted in with slower moving traffic. With rail travel growing all over europe, the problems of railways reaching saturation point has forced new lines to be build. This is why despite the success of the Italian Pendolini a new high speed line with 185mph trains is being built, because existing lines are at saturation point.
 
 

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