This is probably the best option until online bookings resume. These agencies will book trains for you and have the tickets waiting for you at your hotel in Bangkok. They charge the normal Thai Railways fare plus a 200 Baht handling fee but they will reduce this to 150 Baht if you say you've been referred by seat61.com. You can pick up tickets at their office in Bangkok or have them delivered to any hotel in Bangkok or any other Thai town or city free of charge.
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