Well... This wasn't anywhere near as easy as I was hoping... The Japanese really know how to use every last bit of space in their models and although the full-metal chassis adds a nice amount of weight for smooth running, it is hell when trying to make room for a decoder.
Anyway, that said, everything is in and functioning... there is just a little more to do on making the shell fit back on properly... and a little more to do on lighting. Out of the box, the locomotive does not have reverse marker lights... just the top front headlights. So I'll be purchasing some 3mm red LEDs tomorrow to fix this situation (there is just enough room to install them.)
So, after dismantling the locomotive, reality hit me when I realised how little room there was inside. The chassis is a two-part construction with each side conducting either negative or positive. This was bad... the power came directly off the bogies and into the chassis and then straight to the bushes on the motor. I had to insulate this somehow and get wires in there. I managed to drill through the soft metal chassis and then solder on to the bushes (There are actually removable contacts that slide on, of which I soldered to.)
In the picture above you can see that the chassis fills the entire shell and there is only a few chunks missing where the headlights go. Since I no longer needed the diode/capacitor combo for the original headlights, I could remove the board and use one of the spaces for the decoder.
I went and bought a set of files and got to work at hacking out a space for it to go. After I'd removed enough I decided to quickly tape up the wiring and decoder and attempt to run it...
Well, no problems here... the locomotive ran like a charm... at some very very slow crawls! The Hornby 8215 decoders (although cheap) contain the back emf technology for really low and strong speeds.
So, the next step was lighting... I unsoldered the original 12v globes from their boards and wired them directly to the decoder... they then were taped in place where they used to sit.
There was not much room at all near the decoder, as it had taken the spot of the previous light. In the end it's still too tight and there needs to be a little more room made (more filing.)
These locomotives were not fitted stock with reverse marker lights! They have the clear plastic inside and just enough room to hold a small globe to light them... so I haven't completed this yet until I get them working...
And then I need to find the carriage set for the my Twilight to pull :)