Here's an alternative way of wiring and fitting decoders - I use a plug and socket with standardised wiring for the decoder. The white block is a 4-way plug & socket but an 8-way one would deal with lighting. This is in an ALCO SD40T-2 model
If I want, or need, to run the loco in DC-only mode all I have to do is unplug the decoder and fit the jumper unit instead. (It also means that I can upgrade decoders relatively easily in the future.)