I have a tiny Accucraft Porter switcher loco. There is nowhere practical on the loco to hide and install battery/sound/RC. So, I converted one of their small mining wagons into a tender to carry all the kit. Some of these diminuative Porters ran with tenders, I did a Google search. A small low profile rectangular speaker is mounted under the wagon, between the frames, the rest of it in compartments inside. I am limited to AAA size NiMH batteries, but as it is a small loco used for light duties this is not an issue. However some use LiOn types, for their compact size/power, to overcome this issue. I then just have the two wires from the motor feeding back to the tender. You could have the wiring for lighting and smoke too if you wanted.
I use a trailing battery only wagon too for installations were large battery packs cannot fit easily, with the Rx/Sound/switch/sockets on board the loco. I have power leads at both ends on those locos that are of the geared non tender type. Max
Mining car. Coal load and water filler drop in. Charge socket and power switch in water filler. Speaker under car between frames..
View attachment 348914
Loco and converted mining car. Loco is 1:20.3 and really very small with no spaces to hide and fit things in.
View attachment 348915