Rail infrastructures are massively subsidised though. Apparently subsidies for rail in Germany were €17B in 2014.

Though a carbon offset tax would only increase ticket costs by around €2 per leg.

That depends on the tax amount, doesn't it?

The rough estimate of the amount of damage a tonne of CO2 released into the atmosphere causes is $196 (source: wikipedia).

A an economy class return flight from Munich to Barcelona (a typical night train distance) releases 0.45 tons of CO2 per passenger resulting in an additional cost of $88.2 (72.6€), 36.3€ per leg.

Damage costs != offsetting costs

Every extra tonne of CO2 does cost $196+ (I’ve seen credible estimates as high as $400), but through offsetting you could verifiably ensure that more tonnes are avoided for far less.

As a crude analogy, if there is an oil spill (causing $$$ of damage), it would still be cost effective to plug the next oil spill for $.

Don’t get me wrong, the ideal scenario would be to not cause the first oil spill at all (in this crude analogy), but given that this seems impossible in our society, an offsetting tax on plane tickets could be a highly cost effective lever instead.

At which price point for CO2? Current estimates of >70€ per ton could make a flight from London to Berlin about 10€ more expensive.

It's $20 in Canada (where a carbon tax exists) and that comes out to 12 euro