I think “train autism” means train spotting (the hobby, not the movie). In the hobby, you observe trains and write down their model and/or serial number in a notebook. The goal is to get a full set of trains of a certain kind, or operated by a company. Nowadays the community also posts photos on the internet, but for a long time, they were known as the guys comparing their notebooks with nothing but numbers.
The numbers were code for emperor Nero. If you assign every letter in the alphabet a number based on the alphabetical order and add up the values for the name, you get 666. But only if you use the Greek name NERON CAESAR, in Latin, he would be NERO CAESAR, and the missing N gives you only 616. So some scribes in the western part of the empire would calculate in their head and write down 616. So we got two versions of the number going around in manuscripts from the time.
That’s why the dwarfs where helping him with his decision.