We inquired during our last stay (Nov05) because we were to depart during Loy Krathong festival and I epxected very heavy traffic.
They said they didn't offer it at all at that time.
Not sure if it's just not available after dark or just not during Loy Krathong.
Anyway, traffic ended up being no issue at all and we made it to BKK in under 40 minutes.