the trouble with trolleys
Sep. 8th, 2004 05:01 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
totientAt the Worldcon, I got into a discussion with someone about how come the low-floor trolleys are so unreliable compared with low-floor buses. Here's my understanding of the answer.
First, consider how ordinary trains turn. Trains have metal wheels which exert force well in the normal (ie, down) direction, but poorly in other directions. Trains also have solid metal axles, which means pairs of opposing wheels turn at the same RPM. Finally, although trains have flanges to keep them from going off the track, any time the flange actually bears force on the rail it causes an enormous amount of friction and wear. To solve these problems, train wheels have conical bearing surfaces. As the train gets off center on the track, the outside wheel will ride up the shoulder of the cone, and the inside wheel will ride down to the smaller diameter portion of the wheel. This makes the train tilt slightly, changing the direction of the normal force, but more importantly it makes the outside wheel travel more distance per revolution, allowing the train to turn, without making the flanges rub against the track.
Low-floor trolleys have stub axles. This is bad not just because stub axle bearings have to be able to take torque, but because the turning mechanism described above doesn't work, and something else has to replace it. Low-floor buses also have to deal with stub axle bearing design problems, but buses are lighter than trains and more importantly they have cylindrical rubber wheels which can exert sideways force without undue wear.
All of which is why the Silver Line is a bus instead of a trolley.
First, consider how ordinary trains turn. Trains have metal wheels which exert force well in the normal (ie, down) direction, but poorly in other directions. Trains also have solid metal axles, which means pairs of opposing wheels turn at the same RPM. Finally, although trains have flanges to keep them from going off the track, any time the flange actually bears force on the rail it causes an enormous amount of friction and wear. To solve these problems, train wheels have conical bearing surfaces. As the train gets off center on the track, the outside wheel will ride up the shoulder of the cone, and the inside wheel will ride down to the smaller diameter portion of the wheel. This makes the train tilt slightly, changing the direction of the normal force, but more importantly it makes the outside wheel travel more distance per revolution, allowing the train to turn, without making the flanges rub against the track.
Low-floor trolleys have stub axles. This is bad not just because stub axle bearings have to be able to take torque, but because the turning mechanism described above doesn't work, and something else has to replace it. Low-floor buses also have to deal with stub axle bearing design problems, but buses are lighter than trains and more importantly they have cylindrical rubber wheels which can exert sideways force without undue wear.
All of which is why the Silver Line is a bus instead of a trolley.
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Date: 2004-09-08 03:44 pm (UTC)no subject
Date: 2004-09-08 04:59 pm (UTC)no subject
Date: 2004-09-08 09:55 pm (UTC)I don't think the problems with the Bredas are particularly related to the stub axle design. There are numerous other low-floor trolley designs out there that work just fine. The Bredas, like any other vehicle for the Green Line, had to be pretty much a custom job due to the tight tunnel clearances and the requirement to be able to trainline with a Type 7 (so that there would be one accessible car in each train). One theory on the cause of the problem is weight distribution-- supposedly the center truck is unusually light compared to the ones at either end, and it's the center one that tends to derail. Breda claims the problem is the T's crappy track, and in fact the T had been spending a lot of money having the rails reprofiled.
I don't think there's any intrinsic problem with stub-axle vehicles-- either type is going to ride up the outside rail and lean into turns; this effect is unrelated to the relative wheel speeds. Solid-axle card depend on this effect to avoid slippage; stub-axle ones don't. Talgo has been making low-floor stub-axle mainline passenger cars for many years; Amtrak's Cascades service uses them.
As for the Silver Line, a big part of the reason it's a bus and not a light-rail line is that the US DOT was on this "bus rapid transit" (a complete oxymoron) kick and was more willing to fund the one than the other. If it was a light-rail line, they could have run it into the now-disused Tremont Street tunnel and had it join the Green Line at Boylston (That would probably required modernizing the signal system to handle tighter headways, but they need to do that anyway.
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Date: 2004-09-08 09:56 pm (UTC)I think counting the Breda experiences as an explanation of why the Silver Line is not rail is a bit of a red herring. The Breda issues came up after they'd already started ordering articulated buses and putting in street furniture for the Silver Line.
The promised line to Dudley Square got attached to the South Boston Transitway project to give one line two endpoints; I've seen references to a different busway project (the Midtown Connector) which would have taken buses on a tunnel under Stuart Street into the Back Bay. That would have given a one-seat ride between the airport, the convention center, South Station and the hotels and offices on top of the Mass Pike.
Given what people actually want to do from Dudley (go downtown and connect), using the old Tremont Street portal and attaching it to the Green Line (looping at Park for capacity reasons if necessary) makes a tremendous amount of sense. Washington Street also isn't all that curvy. However, rail in the street is more expensive than the already pricey repaving job on Washington the T paid for, especially to put a reserved right-of-way into the middle, and the T doesn't want to run any mixed-traffic streetcar service they can avoid, ever.
Now, the South Boston Transitway can't be light rail if it's going to go through the Ted Williams Tunnel to the airport. And, by linking the Dudley Square line to the South Boston Transitway, they could take two otherwise unrelated lines they'd decided to build, and use one's limitations to justify saving capital costs on the other, long in advance of finding the funding to build a connector between the two.