If you've stayed around Khao San Road you'll know what a nightmare it is getting across town during the day. However if you're heading in the direction of Pantip or Sukhumvit there's the river taxi.
The river taxi starts from Fort Mahanak - that's the white fort near the democracy memorial - this marks the eastern edge of Rattankosin Island. It stops at Hua Chang, which is good for MBK Department Store and Siam Square; Pratunam which is near Pantip Plaza and Central World; and Nana and Asok - before terminating near Bang Kapi. See Wikipedia link below for a list of stops.
Short journeys cost 10 baht. Important things to remember if it's your first time:
1) The interchange is at Pratunam - all passengers must change here.
2) Try to sit in the middle so it's easier to see the sign denoting which stop you're at. They're very small!
3) Try to sit near the side - less spray there.
4) Note that they often miss stops - for example they often go straight from Pratunam to Nana - don't rely on counting the stops!
5) You must get your orientation right - there's only one landing at each stop serving both directions of traffic.
The history of the Saen Saeb Canal (Khlong Saen Saeb) is interesting. It actually goes out all the way to Chachoengsao near Chonburi and was built to transport soldiers and supplies to help the war effort with Cambodia. Construction started in 1837 and took three years.
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