Is it safe for somebody on their own to travel in london by train late at night?

Presumably you're going by tube to Stratford and then by train. Its not 100% safe, but most of the drunken rowdy behaviour you see late at night is high spirits more than anything else and is not particularly dangerous. Just be cautious - sit in the most crowded carriages etc (I've just looked at your question again. Do take the train from Stratford rather than the bus. Its so much quicker. If you do use the bus, sit near the driver)