We would more commonly use the word "disposal" instead of "disposition" though the two words are synonyms in this context. It means that if Napoleon had the technology and ability to use trains (railroads) for whatever he needed, nobody would have been able to defeat him. Of course, trains or railroads (of the steam locomotive type) were not widely used in Europe until the mid-19th century, and Napoleon was exiled in 1814. Therefore, he was unable to use this technology and was eventually defeated.

The sentence is badly transcribed, with poor spacing and punctuation, I would check where it came from. The original text probably said "at his disposal", which means "available to him" or "at his command".