Conversion to TMCC - for the engines, The Electric RR's AC/DC Commander, which is $140 and out of stock 
. Another option is finding an LCRU kit for powered Lionel GP9s. For the passenger cars, either the MINI COMMANDER II (2 COUPLER CONTROL) KIT at an eye-watering $115, or try to find LCRX kits, install the electronics and sell the couplers on fleabay. Can sometimes be found as low as $30.
Note that that modern Lionel and MTH run both wires of the coil coupler back to the electronics, but Lionel used the frame for ground/outer rail/common, and you can improve things by soldering a wire directly to the frame of the coil coupler, or possibly rewire it it.
I converted a set of Lionel F3s to TMCC first with an LCRU, and later with an AC/DC Commander, but once I tried an engine with speed control it was all over.
I suppose some clever individual could make something using various Bluetooth Low Energy products and some add on circuit AND write an app that works on iPhone and/or Android.
Something I did in the past was hook the horn relay up to the coil couplers in some postwar switcher, but I would get too many accidental uncouplings; maybe it was caused by the crappy grounding.
