It's also from the children's book series called Murderous Math, a group of criminals were bailed out from the prison by a friend who wants them pay back. A solution to get money was offered.
"And where did these ten fat ones (10 million) come from?"
"A friend who wants it paid back."
"Where are we supposed to find that sort of dough?"
"The Fort Knocks Wages Express"
"You're joking, nobody knocks off the Knock Express"
What exactly it is will be determine by the era or by particular circumstances, because there were both express stage coaches and express trains.
In both cases, an express is a transport that goes from one location to another with few if any stops along the way. In this case, it goes to Fort Knocks. (Both trains and stage coaches were frequently referred to by their destination because otherwise there was no way to tell apart all of a company's.) The reason it does not stop is that it is carrying the money to pay wages -- security requires it, especially if they can expect it to carry ten million.
Fort Knocks very likely is a pun on Fort Knox, an American military base in Kentucky.
The cultural reference here is made actually to the United States Bullion Depository. It is a vault sitting next to Fort Knox, but popularly referred to as "Fort Knox".
The United States Bullion Depository, often known as Fort Knox, is a fortified vault building adjacent to the Fort Knox Army Post. It is operated by the United States Department of the Treasury and stores over half the country's gold reserves. It is protected by the United States Mint Police and is well known for its physical security.
The place is known for its safety and tightness. Even in the UK, where your book was originally published, people talk about Fort Knox as a symbol of money safeguarding. For example:
That is why one of the bad guys says: "You're joking, nobody knocks off the Knock Express".
"The Fort Knocks Wages Express" refers to an express train/wagon that carries wages to Fort Knocks for safe storage.
