TCS make small direct plug in decoders with the plug on the circuit board.(They offer a vertical or horizontal orientation).
I have found these decoders control my older analogue models with LFC motors very smoothly after conversion with an ESU permanent magnet.
The factory setting for acceleration and braking is 1 which is a bit abrupt. I change both to 6.

I was converting two analogue locos recently (SBB Ae6/6 and OBB BoBo electric). I wanted to have an engine room light which remained on in both directions but which could be switched off.
These decoders only have F0, but no F1 or F2 so I couldn't use those two functions to control a light.
One way to do it would be to drive it from track power with either a resistor, diode, capacitor set up, or a resistor,bridge rectifier.
Both these use rather bulky components and the light is always on while track power is connected.

Another way is to join the yellow and white wires ( DCC protocol colours) and use that common ground. The blue wire via a resistor is the supply.
The disadvantage of this is that the front and rear lights no longer swap over with direction of travel.

I tried out an idea on the bench and it seemed to work.
The positive supply via a resistor is taken to the anode of the engine room LED and the return cathode is connected to two diodes with their anodes commoned.
One cathode is taken to the white wire and the other to the yellow wire. When the white or yellow wire goes low the diode connected to that wire conducts and the engine room light comes on.
The other wire remains high so the diode won't conduct. The reverse happens with change of direction.






I used two small signal diodes seen on the left of the board as the current flowing was around 12 mA.
I also use the inline machined sockets available in suitable length at each end of the board. Once the socket is filled with solder, it becomes a handy solder point.
This makes it easier to revisit the board if required rather than have wires soldered to the back of the board via the component hole.
The last photo shows how the three leg red/white common anode LEDs fit into the bulb holder. The two lateral recesses for the bulb are perfect for the outer legs. ( covered with heatsink!).

This same arrangement could be used to control a small relay coil with track power via the contacts supplying a smoke generator or other function.The smoke unit would be on when the lights are on.
If your decoder has F1 and F2 then they can be used for other functions. A sneaky way of creating another switchable function.
Cheers,
Howard.