Bachmann OO LB&SCR E4 0-6-2 Tank... a very pretty model that can take a Zimo MX648 decoder and SugarCube7 speaker,
just about squeezing decoder and speaker in the boiler, with hard-wiring and a minor chassis modification.
It is possbile to install a 6pin version of the MX648 without any modification, but the speaker will then have to sit in the cab.
Our example here makes space for everything in the boiler...
Remove the 6pin DCC socket board and the resistors across the motor.
Grind the nugget of metal flat at the very front of the chassis (this is where the speaker will sit)
Drill open the base of the chimney so there is somewhere prominent for sound to escape.
The speaker goes enclosure down, which removes any chance of a short-circuit with the chassis.
The MX648 sits behind it in the boiler. Connect red+black pickup wires and orange+grey motor wires.
Photos courtesy of Richard Allen.
Here is a more complete photo guide of an install done by YouChoos...