The product I was thinking about for a blue-tack replacement is made by Lemax and is called StickyMax. It's a vaselene looking material and about that colour too. It's not really suitable for porous surfaces like paper/timber as it is a little oily like bluetack. I'm going to be using it to attach cars to STVA flat wagons. I'm not sure about a loco's painted surface though.
Might be worth testing first. It only cost me AUD4.40 (GBP1.78).