3 ms·

Now this is reasonably well understood, but it wasn't when BART was designed Claiming that BART was ground breaking and could've never predicted the consequenc

Now this is reasonably well understood, but it wasn't when BART was designed Claiming that BART was ground breaking and could've never predicted the consequences of their design choices is quite revisionist. They got called out for their dangerous train control almost immediately. 80 MPH is not and was not high speed. Amtrak trainsets were cable of over 100 MPH back in the mid-70s. BART just reinvented everything poorly (that's their MO). Anyways here's a technical piece about BART wheels: https://www.bart.gov/sites/default/files/docs/noisereport.pdf https://www.bart.gov/sites/default/files/docs/noisereport.pd... With trued wheels and smooth ground rail on ballast and ties, BART is one of the quietest vehicles. in operation at U.S. transit systems. Problem is BART is exceptionally hard on their track. Even with conical wheels you can feel the new trains bounce up and down on wildly out of round wheels. You can feel the rumbling on the decrepit track between Lake Merritt and 19th St. Normal railways often lubricate their tracks, but BART can't. Normal railways increase the track gauge slightly in curves (because most train cars don't use differentials), BART pretty much doesn't. This was my favorite bit: Of these two types of curving noise, wheel squeal is the more prevalent, while wheel howl may be limited to lightly damped aluminum centered wheels such as used at BART. In fact, wheel howl may be unique to BART, because BART is the only transit system employing rigid aluminum centered wheels. Whoops.

iggldiggl 4y ago
> Normal railways increase the track gauge slightly in curves (because most train cars don't use differentials) Actually gauge widening in curves is to some extent probably more a remnant of steam locomotives with their comparatively ginormous wheelbases. Based on current knowledge, a slight gauge widening on straight track and a slight gauge tightening on tight curves might actually be more beneficial – while trains don't have differentials, they normally (BART and some streetcar systems excepted) use conical wheels, which can self-steer themselves around curves down to a certain radius. That radius is somewhere in the range of a few hundred metres, but its exact value depends on the equivalent conicity of the combination of wheel and rail profiles. A higher equivalent conicity reduces the minimum radius down to which self-steering remains effective, while a lower conicity reduces hunting and increases stability at high speeds on straight track. Simultaneously, given a nominal track gauge, a slight increase in the track gauge reduces the equivalent conicity, while a decrease of the track gauge noticeably increases equivalent conicity.
- inferiorhuman 4y ago
  Based on current knowledge, a slight gauge widening on straight track and a slight gauge tightening on tight curves might actually be more beneficial – while trains don't have differentials, they normally (BART and some streetcar systems excepted) use conical wheels Right, BART got the track gauge wrong when they were using flat wheels. Instead of self-centering the wheels got dragged down the track. This went on for forty-odd years which is how BART earned its reputation for being so damn noisy.

iggldiggl 4y ago

> Normal railways increase the track gauge slightly in curves (because most train cars don't use differentials) Actually gauge widening in curves is to some extent probably more a remnant of steam locomotives with their comparatively ginormous wheelbases. Based on current knowledge, a slight gauge widening on straight track and a slight gauge tightening on tight curves might actually be more beneficial – while trains don't have differentials, they normally (BART and some streetcar systems excepted) use conical wheels, which can self-steer themselves around curves down to a certain radius. That radius is somewhere in the range of a few hundred metres, but its exact value depends on the equivalent conicity of the combination of wheel and rail profiles. A higher equivalent conicity reduces the minimum radius down to which self-steering remains effective, while a lower conicity reduces hunting and increases stability at high speeds on straight track. Simultaneously, given a nominal track gauge, a slight increase in the track gauge reduces the equivalent conicity, while a decrease of the track gauge noticeably increases equivalent conicity.

inferiorhuman 4y ago
Based on current knowledge, a slight gauge widening on straight track and a slight gauge tightening on tight curves might actually be more beneficial – while trains don't have differentials, they normally (BART and some streetcar systems excepted) use conical wheels Right, BART got the track gauge wrong when they were using flat wheels. Instead of self-centering the wheels got dragged down the track. This went on for forty-odd years which is how BART earned its reputation for being so damn noisy.