The best option then is to use a multimeter and with track powered off, check for a sort circuit between from the running rails to each wire. Also, check around the 8 pin socket to see if you have pushed a pin through any wires insulation underneath it, or to the loco chassis itself. Similarly, have a look around and under the Class 50's PCB to check for shorts.
Whatever you do, unless you find and clear the fault don't put it back onto a powered track, a new decoder will just pop straight away
Failing that its a send off for repair.