Can you estimate the additional happiness brought by trains?

No, but we can look at the amount of misery, that is prevented by not using cars. Traffic jams, pollution, accidents, waste of resources,... The difference is so huge, we don't need to put a number on it. Public transport might be unpleasant for the individual, but is clearly very much better for everyone than using cars.

> Public transport might be unpleasant for the individual

Where is this true?

When it’s overcrowded, typically caused by policies such as work from office.

At least here, that only happens the day after holidays.

Yes. For that matter, you can calculate the dollar value of a human life:

https://en.wikipedia.org/wiki/Value_of_life

Numbers vary around the world, but FEMA in the US decided on $7.5M in 2020.

Financial investments are economic decisions. Whether or not you like the idea of assigning financial value to fuzzy concepts like happiness or quality adjusted life years, you still have to do it. The simple act of choosing to spend $N or not choosing to spend $N puts an implicit price on the result.

By putting approximate numbers in the spreadsheet - no matter how crude - we can at least end up consistent and fair. Otherwise we could end up spending vast sums to make a few people a little happier instead of smaller sums to make lots of people a lot happier.