The article briefly mentioned it, but the main reasons these are kept is maintenance and training. That's a far larger concern than the politics, but it isn't as interesting. Hence the nickname "parliamentary trains" -- people love to believe that it's some crazy bureaucratic reason behind them when they're usually very logical.

Some of these routes will probably never be viable on their own, but the segments of track might be needed some day as a detour. For instance if a bridge somewhere needed to be out of service for a year or something.

For example, one of the more famous "ghost trains" (since it's in London) is the daily Paddington to West Ruislip service. Since they're both served by the underground there wouldn't be much need for a mainline train between them. If long-term work disrupted that connection they might want to temporarily run a regular service on that line though.

If you just abandoned the track, you wouldn't know its condition when you needed it. Just as important, the regulators wouldn't let you run passengers using crew that hadn't been trained on that line.

So just running an occasional train and rotating staff through it, the line and the skills are always kept sharp. Then if you suddenly need that bit of track in an emergency it's ready to go.

And since you're going to run a train anyway, you might as well sell a few tickets if people want to join you.