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 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.