Father Christmas,
Provided you have enough wattage to power your trains, you can use your postwar transformer to power a command control environment. However, you should add a modern circuit breaker and TVS diodes. (Some people recommend fast-acting circuit breakers, but the circuit breaker is designed to protect the transformer. If you're using a postwar transformer, an inexpensive thermal breaker will work fine. I would recommend picking up a 3A and a 5A, and if the 3A trips too readily, use the 5A.)
The transient voltage suppression (TVS) diodes are meant to protect the sensitive electronics in modern trains. If you're not familiar with TVS diodes there is a lot of good information on this forum, such as here, which you can find with the search function. The quick'n'dirty is that they are bi-directional and are installed from hot to common or chassis ground. Ideally you would install them inside the locomotive from the center rail pickups to chassis ground, but they will also provide protection if you wire them between your hot and common terminals on your transformer, or at power lock-ons, etc. Most people probably install them at the transformer terminals or at lock-ons.)
An alternative to Legacy/TMCC would be to purchase a Lionel LionChief Plus locomotive, which uses a hand-held, wireless remote, but would not require a new power source—just add a circuit breaker and TVS diodes as described above. You can pick up a LionChief Plus locomotive for starting at a little over $200 for a diesel and $300 for a steam locomotive. Heck! you could even get an E33 electric locomotive for under $200 (see the TrainWorld link). TrainWorld and Charles Ro are two sources, but you might check other forum sponsors or your local hobby shop (LHS), too, for deals and further selection. (The LionChief Plus locomotives are not as feature rich as a Legacy or TMCC locomotive, but they have good sounds and remote controlled couplers. The system is much simpler, requiring no additional infrastructure, other than a circuit breaker and TVS diode(s). You could easily alternate between running your postwar locomotives and a LionChief.)
If you go the LionChief Plus route, for your postwar switches, extend the wires from your remote switch controllers to the switches with 18 or 16 gauge wire. If you haven't done so already, you might mount your postwar switch controllers to a small wood board for easy portability.
—Matt
