A few thoughts / hazy recollections. In the 1970s, cooking in Mark 1 vehicles (RBR, RKB, RK, etc) relied almost exclusively on propane gas (grills, ovens, hobs, Stills boilers, etc). No microwaves or freezers, so no ice cream. Most restaurant car food was prepared fresh, as were buffet car sandwiches. Pre-peeled potatoes were available, but needed final trimming by hand. It was wise to slice onions and tomatoes whilst the train was stationary. Saute potatoes were popular because chips were not possible following the ban on deep fat fryers, which although strapped down to stove tops were heated by a naked flame and could cause horrendous accidents if they ignited or broke loose. Toast was cooked under the grill.
With most meals silver served, fried eggs had to be served on a half slice of toast to stop them sliding off the service tray. If you stretch to roast beef, the Yorkshires were cooked in trays and sliced into portions when done.
If you want to include something a bit unusual, I suggest you include the use of "pomme", a singularly unpleasant variety of instant mashed potato which was carried as a precaution in case of emergencies - happily seldom required!