My 3rd Heljan Railbus = [excluding Kimaster unpowered] having earlier Heljan Park Royal and W+M.
This one bought because it quoted 21pin DCC ready. Still a pain to take apart, but a TXS 21pin decoder does JUST fit with cube speaker in the lobby. Unlabelled bare-ended Heljan wires are their speaker [I worried I had broken a connection at first] connections and should be used for the speaker - but I had opted for the Hornby side socket - concealing the red+white leads in black heat shrink - and passing UNDER the roof weight as there is NO room outside the slots provided for any wiring.
Don't forget the screws under the horns, and where you put the 4 handrails which have to be removed ( as with original ) Would have liked to be able to change destinations - but they are authentic, and now our railbus is populated and sounds great with heritage DMU file (even if too many engines) TXS lighting functions F0 directional lights, F21,F22 high level, and F23 interior lighting F4 spare but unwired. Weight assists good pickup. Still space in the lobby for a stayalive if ever needed.