Intercity 125 - Tilting Trains....

[ToraToraTora]

Been watching the program about the intercity 125 etc. Anyway, they made it tilt to take bends quicker fair enough but the track is not banked it's horizontal to the ground so it's not like a banked race track. The train's undercarriage does not tilt. The top does but this seems to me like it would make no difference. Clearly it does so what am I missing?

[alavahalf]

If you were a standing passenger when the train takes a high speed bend. Were the train upright with no tilt, you would end up being thrown across the carriage and ending up in someones lap. The same same thing will happen to any baggage etc. Imagine the dining carriage, you would have food and drink everywhere without the tilt.
It's all about using the g force to your benefit.

[Buenchico]

While it's all too easy just to throw Wikipedia links at people who're asking questions here, I feel that this one is actually quite good at explaining things:
https://en.wikipedia.org/wiki/Tilting_train

[ToraToraTora]

I know that alavahalf but surely it's primarily about taking the bend faster?? Or are we saying that the tilt is merely a passenger comfort feature so that the train can go faster? I.e. could an non tilting train go as fast but be more uncomfortable for the passengers?

[alavahalf]

It wouldn't be just uncomfortable .. it would be downright dangerous for passengers.

[New Judge]

I’ve still to watch the programme, 3Ts, but the IC125 is not and was not a tilting train. About the same time as the 125 was being developed British Rail was developing the Advanced Passenger rain (“APT”). This was a tilting train but it was not a success. Despite being instrumental in the development of advanced braking systems and electrical traction, overall it was never fully developed to its potential. A press trip from London to Glasgow broke the record for that run but the gentlemen of the press reported that its tilting mechanism made them very queasy. Some reports, however, suggested that this was a result of the copious quantities of free BR booze that Fleet Street’s finest had consumed before the train passed Watford Junction:

https://en.wikipedia.org/wiki/Advanced_Passenger_Train

As alavahalf explains, The purpose of the tilt mechanism is to counteract centrifugal force. Here's an explanation (which is not my work):

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As a train (or other vehicle) rounds a curve at speed, objects inside the train experience inertia. This can cause packages to slide about or seated passengers to feel squashed by the outboard armrest due to the centrifugal force, and standing passengers to lose their balance. Tilting trains are designed to counteract this discomfort.
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Although the APT was introduced into limited public service it did not last long and it was not until the "Pendolino" trains were introduced by Virgin in 2002 when Britain had its first regular tilting train service.

The IC125 was a conventional train in that it did not tilt but it included some novel features (for the time) the most notable of which was a locomotive at each end. This enabled far better acceleration. The trains also had a state-of-the-art braking system which enabled them to be brought to a halt from 125mph in the same distance (about 2,000 yards) as conventional trains could manage from 100mph. This meant existing signalling spaced for 100mph running was adequate for the 125.

Mrs NJ and I travelled on one of the first 125 services from London to Swansea in about 1977. We had a voucher from a packet of “Persil” which gave us a ticket for £1.25! In contrast to the APT's very short commercial life, IC125s (or "High Speed Trains" as they are more commonly known) still abound after more than 40 years service.

[ToraToraTora]

so the train tilts for comfort not to keep it on the track?

[Buenchico]

>>> so the train tilts for comfort not to keep it on the track?

When a fast-moving train enters a curve, Newton's laws of motion dictate that (unless otherwise constrained) whatever is inside it (i.e. both the passengers and their luggage) will continue to go straight on. So, unless the cabin of the train leans over to one side (which helps to adjust the forces acting on the passengers and their luggage) both people and bags will go flying.

The simplest way to get the cabin to lean over is to bank the track, which also helps to relieve the stresses on the body of the train itself and to keep it on the tracks. However that's only possible (beyond a certain limit) if it can be guaranteed that all trains on that track will be travelling at high speed. Such banking isn't appropriate for slow-running commuter services.

So the trickier option of tilting just the cabin (while only providing minimal banking for the tracks) is deployed. The (limited amount of) banking keeps the train on the tracks and the tilting keeps the passengers in their seats.

To complicate matters further though, the fluid inside passengers' ears doesn't move in the same way that solid objects inside the train's cabin do. If the optimal tilting to reduce the stresses on passengers and their luggage is used, passengers will experience motion sickness because of the way that the fluid in their ears is moving. So a somewhat reduced level of tilting is used (being sufficient to keep passengers in their seats but not great enough to result in them all throwing up).

[ToraToraTora]

yeah judge got the 125/APT mixed up. Anyway I have always thought the tilt was to help keep it on the rails whilst going faster on bends and I could never figure out how but now I see it's so it can go faster without throwing the passengers about it makes more sense. So a non tilting train can go faster round bends but it does not to avoid throwing the passengers about?

[ToraToraTora]

all I am fully aware of the physics, I had just focused on the wrong thing.

[New Judge]

The normal constraint on the speed of a train going round a bend is keeping it on the track. An upright train is more likely to "jump the rails" than one that is tilted. Think of trying to ride one of your bikes upright through a corner. You would far more readily fall over - to the outside of the curve - than if you "tilted" into the curve. You can negotiate a bend at a higher speed when you lean into it. The speed limit for a train on a particular curve is determined by its ability to stay on the track rather than the comfort or otherwise of its passengers.