Sound trailers work well on continuous circuit layouts, with or without hidden sidings, because each train is usually running in the same direction each time it runs.
However, end to end layouts are more difficult. A sound trailer can be placed between the loco and it's train and it will be well as it runs from one end to the other. But what happens when the train is reversed to run in the opposite direction; the sound card is now at the wrong end of the train.
On a small, station to fiddle yard layout, the best solution is probably to use a layout sound system under the baseboards. By using multiple speakers, which can be switched in and out, the sound can "move" with the loco.
For a sound system on a larger layout, with a branch line and terminus, then a sound trailer should work well in an auto-train since the loco always stays at the same end, next to the soundcard in the coach. In freight trains, a soundcard in a freight car would work as long as the loco always took the freight car with it, even when shunting. The bigger problem is passenger trains made up of several coaches which could be pulled from either end after the loco has been turned and "run around" the train at the terminus to pull the train in the opposite direction. A solution would be to install a "reversible" sound system in each string of coaches, as described below.
The method involves installing the soundcard in any of the rolling stock in the string. However, an additional speaker is purchased so that one can be located in each end of the train. These speakers are connected to the soundcard by a pair of thin wires running from coach to coach down the train; perhaps the type of wire used to connect DCC decoders. A double pole double throw (DPDT) relay and diode (IN4001 or similar) is then used to switch in one speaker or the other depending on the polarity of the track supply and hence the direction of travel. In fact, if this solution is adopted then it might be worth fitting coach lighting at the same time with LEDs in each coach powered through another pair of wires from the soundcard battery. The diagram above shows both reversible sound and lighting.
In still larger, end to end, layouts, with main line termini, station pilots would be used to release incoming locos and reposition strings of coaches. In these scenarios, a soundcard in each loco is probably the only realistic option.
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