ok, it is absolutely do-able though many will tell you you are crazy!
will you be taking luggage? a buggy? anything else?
simple practical tips to keep the journey happy. pack each child a small bag, with toys that they like, I tend to use toys like these www.elc.co.uk/toy/world-people-set/ as baby (11 months) can chew on them and wave them around but the older 2 (3 and 5) can play more with them. i also take a pencil case and colouring book each and a couple of pots of snacks and bottles of drinks each. if you have all these things put by individually then theres no shes got mine etc etc.
getting on and off trains for connections, give them plenty of notice, find out how many stops on the train and count them down, when you leave the station before do coats, shoes, back in buggy etc etc if older child is walking a rope or dressing gown chord or something she can hold onto but have a little distance is useful, my buggy has a strap that mine hold but im aware most dont.
to be honest with a bit of planning its pretty easy and i have been doing it since eldest was a baby, have done two 8 ish hour journeys both with 3 stops in the last 6 months on my own with them and despite having the comments about my sanity have actually quite enjoyed it, though was relieved to arrive each time. I actually find the younger two easier to travel with than the 5 year old who is constantly asking questions at awkward moments!
If you will be taking luggage make sure anything you can take can be removed from the buggy, that way you can aim for the wheelchair spot but you are prepared to fold if someone needs it, using that spot is very handy but obviously a luxury.
also look into tickets, with my eldest i found it was often cheaper on a long journey to pay for her ticket on a family railcard as well as mine than to just pay full adult fair for me, that way also you can reserve her a seat so if you do decide/need to find seats you only have one to go on your knee!
sorry this is a bit of an essay, hope its helpful though