Is passport number really necessary? Can I change passport details it after purchasing tickets?

Yes, passport numbers are required. You must also show your passport when you pick up your Thai train tickets.
However, if you need to change the passport details after purchasing the tickets, you can do it for all bus, van, and ferry operators.
You can also change the passport details for some train bookings.

Tags: Thailand, Thai Railways

Thai Railways

OK

I don't have a booking number. I have not purchased a ticket yet.