Hua Lamphong Station is only 20 minutes by Taxi from Khaosan Road and it cost about 100 THB. However, journey back (from Hua Lamphong Station to Khaosan Road) could be little bit more expensive ;o)
I traveled from Hua Lamphong Station to Chiang Mai over the night and it’s was the most comfortable night train. http://en.wikipedia.org/wiki/Khaosan_Road http://en.wikipedia.org/wiki/Hua_Lamphong_Railway_Station