Hi mothercake,
Didn't want your post to go unanswered! I'm sure you will be fine
I did lots of train-ing (and coaching - would not recommend!; and plane-ing) alone with small person from a few weeks to assertive toddler stage... Trains are my favourite way to travel with a small person. Mine much preferred it to being trapped and bored in the car. I'm really sorry to hear about your friend's luggage being stolen - I hope that is a very rare occurrence!
Make sure you remember to take plenty of nappies, wipes, food, lots of snacks, water etc (for you as well). Not sure whether your little one is walking? If so that should keep them entertained for a while...! Up and down, up and down. If still crawling I have been known to spread a plastic sheet in part of the vestibule for them to roll/play a bit. I'm a health and safety nightmare, obviously! A couple of small books, some small toys (you may find good suggestions on some of the airplane threads), I sometimes packed a separate small bag of tiny toys for mine eg a finger puppet, a car, a baby toy, a book for them to unpack and refill etc. I think I found an Aquadoodle thing quite handy at this stage. Will they look at photos on your phone for a couple of minutes?
How much luggage do you need? I tried to eg shove a travelcot (if required) under the buggy and then carry a rucksack/holdall into the neighbouring seating area with me and small person.
Weirdly I don't recall ever changing in a train loo (except Eurostar when taking her with me for work), so obviously got away with it on domestic legs! I have been known to change child on top of the buggy (with a huge changing mat) in places where no toilet was available. You could always wheel the buggy into the disabled loo I guess and do that if you were really uneasy about the changing facilities?
Hope this helps.