No probs,
Spray the primer, Used some pre-shading using Black and white, You can just about make out the white areas coming through.
Then used a Sponge with Maskol and dabbed it over the Gray (did not want to use a Metal Colour) Left it dry Spray the yellow and rubb off the Maskol with a Cotton bud. I did not want to over do the Roof as it is a bit worse in the pictures of the real thing.
http://briansolomon.com/trackingthelight/2015/08/19/irish-rail-weedsprayer-at-heuston-station-18-august-2015-three-photos/irish_rail_074_w_weedspraying_train_at_heuston_dscf5054/
The white pre shading brings up a slightly different hue yellow as indicate in the ring below, Give a Small bit more interest in the overall Yellow.
I added Top hats to the Bogies too for that bit of extra glide not that it needed it I'd say.