“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!