Gynvael Coldwind - one of the guys from Dragon Sector (but probably not a member of the team that hacked those trains)

wrote an article about why company's line of defence that malicious code could be injected is flawed

https://gynvael.coldwind.pl/?id=777

It is mostly about reverse engineering, compilation process, how thing are laid out in the final binary -.text, .data sections, offsets and stuff like

https://en.wikipedia.org/wiki/Code_cave

https://en.wikipedia.org/wiki/Hooking