I had a very mixed experience with 12Go for a train from Mandalay to Yangon in late July 2018. On the positive side, all documentation was requested well in advance, and the ticket was delivered promptly to my hotel in Mandalay. However, once I boarded the train in Mandalay, I was told by the conductor that the train would not travel all the way to Yangon that day. He offered for me to take my ticket back to the desk to get a refund or partial refund, but with only minutes to go before the train departed I had to quickly decide whether to stay on board or seek alternative transport. I had to get to Yangon by the evening for a flight the next day. In the end, I took the train all the way to the last major stop on the operational route (Taungoo) and found a helpful mototaxi driver at the station who took me to a bus company who had a bus departing for Yangon in a couple of hours. I got to Yangon around the same time as the train was scheduled to arrive, and the additional cost was only a few extra USD. This was all due to flooding in the south that had damaged the tracks. Now, I understand 12Go's terms and conditions that specify they are not responsible for conditions on the ground, only for booking tickets, and I certainly agree that flooding is well beyond anybody's control. However, as the flooding had damaged the tracks days previously, so it must have been known to their agents on the ground that the ticket I was sold would not get me all the way to Yangon, I really would have appreciated a heads up from the company in advance - before getting on the train at 6 o'clock in the morning. An email the day before would have been really really helpful.
The train itself is as reported - bumpy, slow, but absolutely lovely. Much more scenic than the bus, and with more freedom of movement while you travel, and fresh air :)