exactly what I experienced… I am in the train right at the moment. Hua lamphong is still operating for ticket service and a few trains still depart from there. Southbound trains depart all from bang bamru.
I just returned from Hua lamphong. They still run the ticket office and some trains depart from there. If somebody here wants to drive to surat Thani by train. The departure station here in Bangkok is bang bamru.
Apply at least one week before flight just to be safe. But you can apply anytime. Technically there is no expiring date because the Thai pass is tied to your date of arrival.
I arrived here on 2nd February, booked my flight back in November and the Thai pass was approved within a few hours.