SEPTA tested a BREL (British Rail Engineering-Leyland) Railbus throughout their Regional Rail System in August and September, 1985 (I was the program manager). The unit was designated RB004 by BREL and ART-01 by Amalgamated Rail Technologies, BREL's US agent. This vehicle has nothing to do with the 1960s-era experiments with Hy-Rail Buses, noted in an earlier post, or the LEV-2 (Lightweight Experimental Vehicle) operated in NH in the 70s. This was a Leyland transit bus body, motor, and transmission mounted on a British Rail freight car chassis with two fixed axles (no trucks).
You don't see many pictures like this because (1) most of the tests were conducted between 11:30PM and 5:30AM, except for limited daylight runs to Newtown, Pottstown, and Thorndale, the vehicle was stored out of sight inside Lenni Shop between runs, and (2) the Philadelphia newspapers were on strike during most of the test period. The intend was, if successful, railbuses would have been used to restore service through a private contractor to Newtown and Pottstown.
As some posters remarked, the melding of bus and rail components was a mechanical nightmare. The biggest problem during identified during the SEPTA demo was wheel-slip---it played havoc with the automotive-style transmission (bus wheels don't slip!) which ultimately crapped out halfway through the demo and putting it on hiatus while repairs were carried. Signals were not a problem, however; it carried an on-board exciter like the Reading RDCs and successfully shunted signals and crossing protection everywhere it ran, including during tests conducted by Conrail engineers on some very rusty track on Barbados Island.
After we were done with it, it rolled off to Cleveland (we rode as far as Harrisburg via Reading) then left the country. After a couple decades of service back in the UK, the unit appears to reside in a railway museum. Like everyone else, it has a Facebook page (https://www.facebook.com/messages/RailBusTrust
" onclick="window.open(this.href);return false;)
BTW, I think that's me in the front window to the left of the driver (who sat behind the center window).