“Turkmenistan” magazine had an issue about the country’s railways (PDF) in March 2006, which you can read online in English and Russian.
Turkmenistan is a bit of an information black hole, beyond the legendary revolving gold statue of the late president. There doesn’t even seem to be a website for the national railway company (unless anyone knows better?).
It appears that since 2004 Chinese suppliers have replaced most of the Soviet-era fleet with a range of single and double-unit diesels. I’m attempting to put together a list of the different types they have, but am finding the supplier and official news agency’s numbers don’t add up – if you can help, please do get in touch!
There is no website because internet use in Turkmenistan is among the lowest in the world. They will not speak English/have internet access. This is a very reclusive country and your request for information (no matter how harmless) will probably not be received as it would be in the UK. As of 2011, I don’t think that the new units were not in use because the infrastructure means that the lines are so old and have a different gauge. I have a picture of some current running stock if you can let me have an email address.