I wouldn’t go so far as to say I’m expert at sleeper train travel with a 1 year old, but in the past 6 months we’ve taken 3 sleeper train journeys and 1 overnight ferry, so here’s the quick run-down on experiences so far.
Aberdeen-London, Scotrail single berth compartment: narrow berth, not really enough room for adult + baby (Leon was 9 months at the time). But the bottom bunk is flat and has no gap down the back, so I wasn’t worried about safety. The two-berth compartments are the same style.
London-Venice, Thello, six-berth compartment: this was not the intended mode of transport, but the carriage containing our intended two-berth compartment was broken. (Even more broken, we assume, than the replacement, which is impressive.) The six-berth style compartment we actually travelled in (thankfully not sharing with anyone else, for which I was especially grateful when Leon started to throw up at 3am) had very narrow berths, and the bottom berth was heavily sloped towards the wall, with a small gap between the bottom of the seat back and the bed. I was concerned that it wouldn’t be very safe to sleep with Leon between me and the wall, and it certainly wouldn’t be comfy, so wound up sleeping head-to-tail (until 3am, anyway, when the sleeping stopped). With a younger baby I think I’d have just sat up for the night. I would not recommend this with a baby not old enough to sleep on their own. The four-berth is also this style.
Venice-London, Thello, two-berth compartment. This was much nicer. Bottom bunk was flat, very wide (for a sleeper train; I think 2’6), and had no gap at the back. Lots of room for me and Leon both to lie down comfortably. Would happily do this again, other than the bit where it took him an hour to settle. The three-berth compartments are the same style.
Liverpool-Belfast, overnight ferry, two-bed cabin. Very comfy. Standard size single beds with a rail at the edge (though I still kept Leon on the wall side) and no gap by the wall. Would very happily travel like this again (and indeed I imagine we will, this summer).
If you’re not comfortable with co-sleeping, I think you might be able to fit a small travel cot on the floor in all of the above, but you might want to check with the train company. There is in my experience always room to put a regular sized wheelie suitcase flat on the floor and a couple of inches more than that, so if your travel cot fits that space you should be OK. (I know very little of travel cots so cannot speak further to this.)
I think that, other than the ferry, our sleeper train days may be over now until Leon is big enough to sleep in a bunk on his own, and to see the whole thing as an exciting adventure rather than getting too wound up to sleep…