LAST EDITED ON Feb-24-04 AT 12:22 PM (EST)
Hi Bradhig,to go one step further back than my previous two posters, as this is the point you probably have to start at to follow this traction gang's logic here Trolleys, trams, LRV's, whatever you like to call them, and their systems can be divided into two different "families". The distinguishing element is whether a system and its equipment is set up for single or mono-directional operations, or for bi-directional operations.Systems build for single direction operations are quite common among trolley systems, for a while it actually looked they were going to replace the bi-directional concept almost completely. Single directional systems have reversal loops or - rarely - reversing wyes at each terminus of a route, and often also at major en-route locations where turning around a car/consist might be desirable for operational reasons. Consequently, single directional cars do always travel forward when in line service. This is reflected by the fact that their doors will be on one side only (right hand side if the system travels on the right track, left hand side if the system travels on the left hand track), and similarily the car will only feature one cab at the front end. Of course these cars still do have a reverser and can go backwards as well, be it for shuntnig in the yard, be it in emergency situations when a trolley needs to be backed out of a certain section of the system. Mono-directional cars might even have an auxiliary cab or rather control stand in the back (again, for the shunting), but often of a very basic layout. This does not disqualify them from being called mono-directional, though.Many PCC's in the US, many of the cars on existing Western European systems, cars on almost all Eastern European systems (mostly standardized in the Communist days to use the Tatra-style car which you know as Number1 trams) are single direction cars. Although their systems might have been built as bi-directional systems in the founding days, loops were added later to speed up turning a train around, especially during the days and on systemas where "trains" where the rule, i.e. a motorcar pulling anything from one to four trailers (maybe even more, I'm currently not aware of any system using more than four trailers, but feel free to correct me here)That said, it is almost obvious in which aspect bi-directional systems differ. Their termini do not have a loop or wye, but simply a dead end stub and a switch letting you change over to the other track for the return trip. Consequently, these systems require cars with doors on both sides, and two fully blown control stands or cabs either side. In the early days of the trolley system, this was the standard on almost any system, btw.As mentioned above, the benefits of mono-directional operations made many systems convert there termini with loops, usually over an extended period of time, thus slowly replacing bi-directional cars with new single directional cars route by route. The trend somehow reversed with many of the LRV systems that came and still come into being since the final quarter of the 20th century. A lot of these new systems embrace the bi-directional concept, as in most cases, the new systems use articulated motorcars - MUed to trains if necessary - that are much easier to turn around than a train with unpowered trailers. Furthermore, a bidirectional terminus often eats up less psace than a loop, another factor when re-introducing a LRT system in an already densly packed city.Now, not that I wanted to confuse you... But if you want the full story, then I should also brief you in that there also exists a hybrid form of the two approaches described above. You guessed it right, it is called the semi-bi-directional approach. In these cases, trolley or LRT routes also end without loops or wyes, and have trains reverse their sense of travelling for the return trip. But while the cars have doors either side, they only feature one cab per unit. This means that trains will always consist of at least two units coupled together, with the rear unit "flipped" around to look the other way, in MSTS words. Here's an example:
Picture taken on an interurban style LRT route passing in front of my office: The train consists of three semi-bidrectional cars, with the first two units looking foward, the third one looking backwards (see orientation of pantographs).Hope that gives you the desired answer, and I wish you a nice dayLukas a.k.a Swissie
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