Trevor Hartford wrote:
"Strictly speaking, I don't think the FRA is OK with the use of vacuum brakes or straight air brakes, as there is no failsafe."
Trevor, it is incorrect that 'vacuum brakes or straight air brakes' have no 'failsafe,' assuming that you mean what I believe you do by each of these terms.
The 'vacuum brakes' once used by many American railroads, including most if not all of the Maine two footers at one time, are more properly described as automatic vacuum brakes, in which the reduction of the vacuum in the train line (i.e., the introduction of atmospheric pressure) causes the brakes to apply. "Straight air brakes," in obsolete railroad usage, are essentially the reverse, in that a reduction in the train line pressure directly causes a brake application. Both systems are "automatic" in that an emergency application results from any unintentional break in the trainline, such as the train uncoupling while in motion.
Steam brakes, which are sometimes referred to as "jam brakes," use steam pressure to apply the brakes on a locomotive, similarly to the air brakes used on highway trucks. Neither of designs these would "failsafe" if used on rolling stock coupled together. If the cars were to pull apart unintentionally, the brakes would be completely released.
Early railroad straight air brakes were superseded following the invention of the Westinghouse "triple valve," the ancestor of the K and AB freight car systems and the passenger variants such as PC and UC. In these systems, a reduction in the trainline causes pressurized air stored in reservoirs under each car to be released with great force, resulting in the application of the brakes with considerably more effect and thus facilitating the introduction of larger locomotives and higher speeds.
Trevor Hartford also wrote:
"That having been said, they also have a lot more important fish to fry. It's cut and dry that to meet the legal requirements if you cross a public grossing at grade, you to have automatic air brakes of some sort, this was required by law by one of te railroad safety appliance acts around the turn of the century."
Trevor, you haven't actually cited any regulatory language here. But if the FRA's requirement is that brakes be "automatic," there could be no objection to an automatic vacuum installation. Indeed, most standard gauge preserved railways in the UK use such vacuum brakes, which were standard practice over there into the 1960s at least.
Jon Chase