OK, so totally ignoring my own advice, I've persisted in trying to get the Caboose body shell off, and have succeeded, without causing any damage at all, now it's off I can see why it wont track on my layout.
View attachment 173612
Got the innards PCB board out and found that although the insulated tab was on the truck, the wire had broken, so I removed the insulation on the other side to see what the problem was.
View attachment 173613
You'll notice that both electrical pickup pins butt up against the floor, so that is as far as the truck can travel.
I've thought of four ways to solve this.
1) Flatten the pins down onto the truck.
2) File/cut away the offending plastic to allow more travel.
3) Cut the pins off altogether and mount a DCC wiper to the bottom of the truck.
4) Cut the pins off and solder a wire directly to the pickup.
Your thought's or any other suggestions are welcome.