I am a firm believer in unions. I have been a member of one since I was 16, and was a rep for a while in my last place of work. As others have said, without them we wouldn't have weekends, paid holiday, sick pay or all the working conditions we take for granted.
Striking is never something people do lightly. For one thing strikers lose pay, for another thing, as often as not they still have to do whatever they would have been doing on the strike day when they get back, and in very many cases those on strike care about the people they help (passengers, students, patients, customers, etc). All the same, it is often the only way 'ordinary people' have to fight against losing the conditions that they, or others, have fought for, so when they do it they have my support.
Unions like the RMT can cause chaos, yes. This shows how important their members are to society. People complain when others strike for more pay (although as I understand it the rail strike is also about changes to contracts and job security).
When dentists took millions of people off NHS lists and increased prices to patients tenfold, people might have muttered about the cost of a filling, or cancelled non-urgent treatments; but there wasn't the condemnation of them that the track maintenance workers are facing. Did people cry out about carers on minimum wage being unable to afford treatment to ease the pain of toothache, or those with no money who had to pull out their own teeth, saying that dentists were holding the country to ransom?
There is no difference in morality - both are groups of people looking out for their own standard of living and that of their colleagues, and putting the welfare of their clients below their own. The material difference is that dentists are often self-employed, and can determine their own charges outside of the NHS, whereas the railwaymen are employees who earn wages which are determined by managers responsible to shareholders.
Why is it that when the actions of one group cause suffering it is put down to 'market forces', whereas when the other group do the same they are accused of being greedy and selfish?