This is one of the older models on the railway. The chassis is a single casting and the motor needs to be isolated from it. The model runs OK at low speed, but at high speeds (which are ridiculously fast for this model) the decoder doesn't control it properly: I assume it is getting poor pickup and can't decode speed commands.
A decoder does fit into this model, but it is tight. I used a TCS M1 decoder, located vertically in the cab. I had thought it would fit horizontally in the cab between the shell and the chassis, but it seems not quite to fit. With the capacitor removed, the remaining wires can simply be soldered in place: use the capacitor's wires for chassis and top brush connections.
The chassis opens easily by removing the two screws at the front sides. CAREFUL: don't lose the coupling or its spring. I've used the DCC Supplies"DiGiHat" plastic bush to isolate the bottom motor brush: this is simple to use and results in a quick installation.
Here is the chassis after the DiGiHat has been installed. The plastic DiGiHat bush is under the bottom brush clip, to which the orange wire has been attached. The heat-shrink is threaded over the wire, the wire is soldered to the brass clip then the heat-shrink sleeving is heated up. I use a paint stripper gun for this.
The black wire goes to chassis (use the cut-off capacitor lead); the grey wire goes to the top brush clip (also use the cut-off capacitor lead). The red wire goes under the nut on the thin bolt, still to be re-installed on the right hand side.
Here is the chassis ready to go back; note the red wire attached now to the wire going under the nut.