The obvious one is fuel mainly because of the tank wagons with Shell or BP on them it did not always run in block trains like it does today.
I can't find my rule book off hand but you where required to have two 4 wheel or one bogie barrier wagons in front of it and behind it
These could be empty's or non flamable cargo that does not react with them and defiantly not ammonia nitrate based fertilizer the same behind the fuel tankers.
There was also a set distance it had to be from the locomotive and guards van.so your short two tank train just became five or six not including the guards van it all adds to the fun.
I hope you marshaling your fitted wagons directly behind the locomotive to form the fitted head of the train.
Fitted wagons are the ones with continuous brakes not all four wheeled wagons had them some where just piped.
Explosives is another one can't remember who makes the vans Dapol and Bachmann I think, not sure Hornby has made those.
The rules for those are very strict.
This is where the addition of cards to the system comes in the card has from to wagon type wagon number loaded empty special instructions it works better if your wagons do have different numbers.