On Monday, 12 September 2016 12:51:19 AM AEST Robin Humble via luv-main wrote:
the Montreal metro does that. https://en.wikipedia.org/wiki/Montreal_Metro the tracks drop steeply after each station and then rise again before the next (flat) platform. not sure how much it dips, but I remember it seemed like a lot.
dunno why the idea isn't more widespread. maybe 'cos you need a fair sized dip. back of the envelope says you need a 14m drop to get you up to 60km/h, or 20m up to 72km/h (ideal, excluding friction etc.) mgh = 0.5mv^2 60km/h = 16.7m/s => h=14.2m, or 72km/h = 20m/s => h=20.4m
If you wanted to convert all the kinetic energy to potential energy then you would need a significant rise. But if you convert just 1/3 of it then that's enough to make a significant electricity saving and also a good improvement to start and stop times. The start and stop times are significant to the overall journey time in areas like Glenferrie where there are stations within jogging distance.
with a big dip, if the train stops or slows down at the bottom then maybe it can't climb back up again...?
As noted in the video Michael cited the maximum gradient for steel wheeled trains is quite small. But if the station is 4M above ground level (enough for shops below it) then you only need a 250M ramp at each end, that's about twice the length of a city train.
the Montreal metro is also notable because they have rubber wheels on concrete tracks. very bouncy and noisy, but perhaps it means they can get traction on steeper slopes than steel wheels on rails.
Definitely.
obviously the steeper you can make the gradient and the closer you can put it to the station the longer the train will be kept at maximum speed. a shallow gradient spread out between stations doesn't really help that much - you need the steepest possibly climb/drop to the station to minimise travel time.
I suspect the "big dip" idea is impractical for steel wheels on (potentially) wet rails, and a small dip doesn't help all that much 'cos of the v^2 term.
If you design the gradient to be half the maximum that the train can climb then that will increase accelleration by 50%. Given the very low acceleration of trains (most trains seem to have have less power per carriage than a mid sized car) that will really make a difference.
here's another idea. if Melb trains carried onboard storage (super-capacitors, flywheels, batteries, ...) then they could store the majority of the braking energy and deploy it from on-board storage when leaving the station rather than pulling it from the overhead power cables. presumably trains already do "re-gen" on deceleration, but currently dump it back into the (unhappy) overhead grid?
Trains currently put electricity back into the train grid. In the train grid they have super-capacitors to store power when more trains are braking than accellerating. It's probably more effective to have super-capacitors in the grid as they are heavy and transporting them would reuire more energy, heavier carriage chassis, and probably some safety issues.
but frankly it sounds like improving the Melb trains overhead power system would be a better idea.
There are some serious issues with that. The train company has contracts with power companies for the maximum power that they can draw. They have a maximum draw of about 100MW and changes to the load is a major issue for power stations. On Monday, 12 September 2016 3:00:22 PM AEST Michael Verrenkamp via luv-main wrote:
All this talk of trains and hills reminds me of this simple explanation. Not sure how accurate it is but it has James may so it 'feels' more accurate.
https://en.wikipedia.org/wiki/Rack_railway He makes some great points. One thing he omits is the fact that some trains have gears to allow them to climb steep slopes. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/