Thank you Jonathan.
All very informative.
The motor runs and the light comes on when I apply power to the left and right sides of the iron split chassis.
However train does not run on the track, because there is no power getting through.
That said however, today I applied power to the driver and pushed downwards (loco was bottom up), and I did get power through and the wheels turned and the light came on.
This tallies with what you said, so I will persevere now and get this thing running!
I know it's old, but it's in excellent cosmetic condition and it only cost me about $50!
Thanks again.

Hi everyone,
This may seem like a silly question, but I cannot see anywhere that there are pickups on this loco.
It is a standard GS4 with split chassis, and non-metal tender wheels.
There are two very small brass washers on the metal part of the axles of the front driver.
It appears that these SHOULD make contact with the LH and RH sides of the iron chassis halves, to convey the current to the motor, but they do not contact the chassis halves.
Is there something missing here, or should there be another system of pickup which might have been removed?
If all else fails, I suppose I'll have to make up a custom system of pickups.
PS the motor turns fine. Also these brass washers don't seem to appear on the exploded diagram of this loco.
Thank you in advance!