Nha Trang Railway Station is located in the city center. It's convenient to get there, and it's easy to buy a ticket at the ticket office. They understand English well. Trains with different classes range from regular seating (there are upholstered chairs) to excellent compartments. The last time I went was on January 25th.
The place is atmospheric, do not build high expectations, and a trip from Nha Trang to Ho Chi Minh City is a special pleasure to see enough color
The train may be delayed - this is normal