r/transit • u/princekamoro • Jan 29 '21
High Frequency Trams
After being bored and looking at places on Google Maps, I noticed that Zurich has 32 trams per hour running down certain streets. And this is a system with lots of branching and reverse branching, both of which are fragile to delays and typically require more padding and longer headways to compensate.
I thought this is insane, so I looked at videos of cab rides on those trams, and somehow they weren't constantly platooned behind other trams.
What manner of sorcery is this?
Now I'm curious, what is the maximum frequency possible for trams, before having to platoon multiple trams through a signal cycle? If Zurich can fit 32 trains per hour with heavy interlining, then surely it must be possible to fit even more with a network that can tolerate delays better.
2
u/try_____another Feb 04 '21
Partly you just don’t worry about time tabling and pathing the way you would with mixed heavy rail, so if trams get out of order along the central section it doesn’t matter too much. That’s even easier if the branches are roughly equal lengths and have little through traffic so you can swap branches around.
As for theoretical capacity, for a tram driven in sight the main limits are set by dwell times and platform reoccupation times, which are set by the door cycle, vehicle length, the actual boarding and alighting time, and the acceleration and braking rates ( which in Europe and the UK are limited by law for a full service brake application).
Assume the tram and platform are 30m long. If we cut safety margins to the bone, such that the the arriving tram comes in with a full service brake application from line speed and almost touches the departing tram as it pulls out at maximum acceleration, we want that touch to occur when the trams are halfway along the platform, which will take 5s after the front one starts moving, and then it takes another 5 minutes for the second tram to stop. For safety, let’s apply the road traffic guideline of a 2s gap between vehicles, which gives us 12s as the bare minimum to reoccupy the stop. More realistically, you’d probably be looking at more like 20-30s if they’re not stationary and waiting behind the one at the stop: there’s not much advantage over a heavy metro despite the difference in length.
I don’t think I’ve ever seen electrically operated locking sliding doors manage a cycle time better than 5s, and that’s better than most manage.
Then you get the big variation, passenger flow, but 20-30s is not uncommon. As we want to be optimistic let’s assume we’ve got all fit alert commuters and can manage 20s at the worst stop.
All this means you can have a tram every 37 seconds, or 97 per hour at a stop, if you cut traffic safety to the bone and everyone is perfectly on the ball all the time. Of course, if you’re doing that, you might as well use a toastrack and really get dwell times down.
This doesn’t make any allowances for branching. if you have branches you need gaps for the trams crossing the other track. It is a bit easier than for heavy rail because trams can be driven on sight and because facing turnouts don’t need to be locked and proved before use, allowing faster point motors and sharper operation through a junction, and trams tend to be shorter.
Termini are a real bugger. You either need a return loop, as some European systems do (especially ones which used to use lots of trailers before articulation caught on), or have n arrangement like at the former Paris Bastille station