Right, this is where it gets complicated.
If I was specifying the cream colour for a print job, it would be something like 15% or 20% tint of process yellow. That wouldn't be ideal for printing your own decals because you need a solid colour.
It took some while to locate suitable solid colours in the Pantone Matching System, but the nearest I could get to was 7499 or 7506. Most of the other yellows waver between lemon through to pink via green.
I hope that helps.
By the way, steam loco cab side numbers were the same colour, outlined in black. I can't verify this, but I believe the coach lettering and numbering had a thin black outline as well.
One more thing, don't forget the printer will make the colour up from the four inks, so the colour may be well off from what you expect. 7499 is the only colour that splits into the four colour print system (cyan magenta, yellow and black) the best.