Still a bit sketchy here, but Ertl are well known, they make toy cars and other vehicles, including trains, and these cards are said to have been on the back of the packaging. Now we do know that Ertl made models of Thomas the Tank Engine and his Friends, and that they were sold in blister packs, where you got a card, and a plastic bulge fitted to it which encapsulated the model, but I have yet to find a packet with a card printed on the back.
However the cards were also sold in packets, eight cards per pack.
I now know that there were 60 cards in this group and they were billed as being "Series 1" but there was no series two, no idea why. There were also slightly different cards in solid colours, which seem to be called activity cards.
There was also another set, issued earlier, in 1992. These were definitely on the back of the packaging of the trains, I have seen them, but they do not feature Sir Topham Hatt on the backs and they have the number of the card on a carriage lamp, the number of the card being on the circle of the glass. Highest seen (in the ten minutes I have had to look) is number 34.