I would assume that even if this woman was never planning to travel by public transport, she must have considered the possibility of encountering stairs when out and about with her buggy, and should have brought a buggy that she could take up and down stairs if necessary - but maybe this is an unfair assumption.
I had lodged with a friend who had 5 children, and had been out and about with them a lot, using buggies and prams, so when I got pregnant the first time, I knew what things were important to me, but if you've never actually had to use a buggy for any length of time, I can quite understand that you mightn't realise how important it is to get a buggy that you can fold easily (with one hand, in case you have a second child later on), that you can get up and downstairs alone, and (for me) that has swivel wheels at the front, so you don't do your back in heaving the buggy round corners!
We never asked for any assistance when buying any of the buggies we used for the dses, so I don't know how much advice expectant parents get from shop assistants, nor how useful it is - but on more than one occasion I have seen couples being shown a huge buggy with fixed wheels, and have wondered if I should go up and share my 'wisdom' with them.
I do think she was being a bit of a princess, as others have said. If she had had any health issues that meant she couldn't carry or bump the buggy up the stairs herself, then I am sure the Sun would have mentioned it, as it would have made the station staff look even worse. I do also abhor the apparent culture of complaining to the press before speaking to the organisation concerned, and giving them an opportunity to apologise/sort the issue out.