There is already an outer ring road (the Kanchanaphisek Outer Ring Road) in various stages of completion that consists of highways. This page examines proposals for an outer ring road that also includes train and/or Skytrain services (the idea being that existing inner city mass transit systems would stretch out to the ring road) as well as ideas for the Industrial Ring Road which includes a giant bridge over the Chao Phraya River. The latest incarnation of the Ring Railway project (May 2003) envisions tunnels under the Chao Phraya instead of bridges. Also contract after contract granted to build roads that would lead up to this bridge area have been cancelled over irrgularities so it is unclear if such a bridge will become reality.

Thawatchai Wongrach sent in this great link to the official website of the Industrial Ring Road Project (January, 2003 - The link used to be there but now it is dead.). It is only in Thai, but the drawings are definitely worth a look--including what would be the biggest bridge in Bangkok. Click on the map in the bottom right-hand corner of the page for full-size map of the project.

Ring road nears completion - B8.7 billion project now 80% complete - Bangkok Post, April 17, 2006
...The industrial ring road is designed especially to improve transport of products from industrial areas on both sides of the Chao Phraya river south of Bangkok to Bangkok Port in Klong Toey and other parts of the country.
The ring road can serve up to 100,000 vehicles a day and is expected to relieve traffic volume on the nearby Rama IX bridge by 50%.
With the industrial ring road, cargo trucks will not have to run through congested districts of Bangkok to reach the industrial zones in Pu Chao Saming Phrai, Phra Pradaeng and Suksawat areas. Besides, ferry service across the Chao Phraya river in Phra Pradaeng will become a thing of the past for most of the trucks...

Industrial Ring Road is 31% done - translated and summarized from Dailynews, October 13, 2004
A report from Rural Highway Department said that the Industrial Ring Road is 31% done and can be described as follows:
1) Southern Section (bridge across Chaophraya at Southern Phra Padaeng with 7 traffic lanes, 702 m long and 1090 m ramp along with the interchange at Poochao Samingphrai Road): 27% done
2) Northern Section (bridge across Chaophraya at Northern Phra Padaeng with 7 traffic lanes, 576 m long and 2300 m ramp along with the connection with Rama 3 Road): 26% done
3) Western Section (interchange in the middle of the bridge and the 1368 m ramp along with the 70 rai public park under the interchange and museum): 41% done
This Industrial Ring Road is for transportation between the industrial estates in Prapadaeng and Samut Prakarn and Klongtoei Port and the connection to Rama 2 Highway, Sukhumvit Road and Bangna - Bang Pakong Highway with the total price tag of 8.7 billion baht. This will be done in November 2005 and ready for service on December 5, 2005.

Pushing the Ring Railway again - May 26, 2003
It was just over two years ago that Bangkok Governor Samak first introduced the Ring Railroad that would encircle Bangkok. It also had a brief resurgence in the news about a year ago with promises of Japanese of Chinese government money. Now the project is showing signs of life again.
Comments: Khom Chad Luek Daily (May 24, 2003) reported that Krungthep Thanakhom has held a Public Hearing for Bangkok Ring Railway.
The preliminary budget of for the 82.5 km, 37 station Bangkok Ring Railway has balloon up from 43 billion baht to 58 billion baht, and still growing, since inflation and interest rates have not been put into account yet.
88 billion baht still be a large sum of money indeed (about 2.5 times of BMA annual budget). The largest expense comes from an underwater tunnel and land expropriation around Kaset Intersection and Bang Kapi intersection.
The connections with the Mall Bangkapi, the Mall Ngam Wongwan, the Mall Bang Khae, Seacon Square, Seri center, Carefour Sri Nakharin, Phra Padaeng Pier, Phiboonsongkram 2 Pier (near Rama 5 Bridge - now with the road to Kanchanaphisek Outer Ring Road - the road to Rattnathibet and Phetkasem Road are the next), Bangkhaen Railway Station, Hua Mark Railway Station, Happyland Flea market (the first flea market which accepts major credit cards - for business survival of course), Phongphet flea market, are also another set of necessary expenses to ensure convenient connections for passengers.
Extra money is also needed for interchanges for the future Yellow Line at Bang Kapi, for first section of the Orange line at Lam Salee and for the second section of the Orange line at Suksawat, for the Southern extension of the Subway at Bang Khae, for Samrong Extension of Skytrain at Thepharak, and for Kaset Extension at Kaset. BMA should set a side the budget for the interchanges - especially the ones which connect the Skytrain with Bangkok Ring Railway.
That the construction could be done three years as claimed is really incredible. Even the goal of 5 years is very amazing. 7-10 years are more convincing.
While planners have taken heat since this project was never part of any masterplan and was originally proposed as a 'done deal' set up with foreign companies by Governor Samak, it does have the advantage of creating a framework for Bangkok mass transit (systems having the goal of their lines reaching the ring) and such a ring would ensure a flow of passengers for current and future projects.

Ring Road/Ring Railway News 2003
Ring Road/Ring Railway News 2002-2001

There are various large bridges roads planned for south of Bangkok, but, according to this article (21-km Bangkok southern bypass to be reviewed - Bangkok Post, February 5, 2003), it seems nothing is really settled yet.