But a penalty fare is a very focussed response to such a situation. If one of the issues is that staff have very few levers when a passenger refused to move, then saying that such a lever should not be used because some passengers might be irritated at it doesn't move the situation forward. Passengers are often irritated at penalty fares in other contexts. It isn't used as an argument not to have penalty fares.
As for the 'this is the only time I can see my kids' argument, no one is saying the other passenger shouldn't remain on the train, only that they need to get out of occupying that space.
Good luck issuing a penalty to someone on a packed train, where there’s limited room to move, surrounded by stressed and pissed-off people all laden with large amounts of luggage and quite likely plenty of kids, many of them not regular rail users - and do all of this in a station stop time without delaying the service.
So you’re potentially asking someone to vacate a space, move to somewhere else that doesn’t exist (ie is already full) or move their luggage to a luggage rack that is probably already overflowing, someone that’s already exasperated because they’ve already not got a seat and had to occupy the disabled space for however long already.
As I say, it’s all very well talking about what might be feasible on a quiet train where everything’s fine and dandy and someone’s just being awkward because they simply fancy using the disabled space. An overcrowded train is a completely different ball game. Add in a bit of stress and emotion to the situation and these seemingly easy solutions simply don’t work.
I’m not saying any of this is ideal, it’s just reality, and won’t change as long as we have overcrowded trains.