Here's a handy map of the region. The current railhead is at HCMC (Saigon), which can be reached from China either via Nanning or Kunming. Both links are (AFAIK) already operational despite being shown as dotted on the map.
The most likely candidate to be completed fairly soon is the Cambodian route, which has gaps in the network between Saigon and Phnom Penh, and again between Sisophon and Aranyaprathet (shown as "Poipet" on the map, which is the Cambodian side of the border). The Cambodian railways are in terrible shape and there are no passenger services between PP and Poipet at the moment, but the Cambodians have already been donated engines, tracks etc and all they pretty much need to do is, um, start doing something which them.
The second option is from Vung Anh to Nong Khai in Thailand. The first stretch of this from Nong Khai to Vientiane is actively under construction and may open as soon as 2008; the second (much longer) bit from Vientiane to Vietnam will take rather longer. Vietnam is pretty keen on getting this done though and, as the Lao PDR is buddy-buddy with their fellow Viet commies (unlike the much-invaded and perpetually suspicious Cambodians), this seems more politically likely.
The other dotted routes on the map through northern Laos and/or Myanmar into Thailand remain pipe dreams at the moment.
jani
In more detail...
Here's a handy map of the region. The current railhead is at HCMC (Saigon), which can be reached from China either via Nanning or Kunming. Both links are (AFAIK) already operational despite being shown as dotted on the map.
The most likely candidate to be completed fairly soon is the Cambodian route, which has gaps in the network between Saigon and Phnom Penh, and again between Sisophon and Aranyaprathet (shown as "Poipet" on the map, which is the Cambodian side of the border). The Cambodian railways are in terrible shape and there are no passenger services between PP and Poipet at the moment, but the Cambodians have already been donated engines, tracks etc and all they pretty much need to do is, um, start doing something which them.
The second option is from Vung Anh to Nong Khai in Thailand. The first stretch of this from Nong Khai to Vientiane is actively under construction and may open as soon as 2008; the second (much longer) bit from Vientiane to Vietnam will take rather longer. Vietnam is pretty keen on getting this done though and, as the Lao PDR is buddy-buddy with their fellow Viet commies (unlike the much-invaded and perpetually suspicious Cambodians), this seems more politically likely.
The other dotted routes on the map through northern Laos and/or Myanmar into Thailand remain pipe dreams at the moment.