I don't think this would effect tourism since foreigners that use these trains tend to be budget travelers generally buying the limited sleeper tickets.

The bulk of the passengers are Thai. As memory serves me, Thais have in the past and may still be able to ride for free. So where is the motivation to spend money?

Thais can still ride for free, but I understand it's only in 3rd class.

IIRC they can only take local-trips, not end-to-end ones, a few years ago I was surprised (and delighted) to get a B75-ticket on a 3rd-class-only rattler from Den-Chai to Chiang Mai for free, but was told that they would have taken my money had the trip been any longer.

This was, like the free-electricity for very-low-useage-customers, one of the measures introduced to help cushion the impact of the global-economic-crisis on the poorest Thais.

Is this the primary rail line to Chiang Mai? I am heading to Thailand next week and would like to visit parts I haven't been to yet. The overnight sleeper to Chiang Mai is on my list of possibilities.

Is this the primary rail line to Chiang Mai? I am heading to Thailand next week and would like to visit parts I haven't been to yet. The overnight sleeper to Chiang Mai is on my list of possibilities.

Yes, this is the only line, to Chiang Mai, but not to worry ...

... buses & planes will be operating normally, and so will the trains, but only as-far-as Uttaradit.

Have a good trip anyway !