I took the train from China to Laos on Jan 1. While the entire line is Chinese-built, you can note some differences - speeds are comparable (120-140 km/h), many km of tunnels. Phone service on the Chinese side is 5G, phone service on the Laos side is spotty at best.

Security - had to get re-screened after going through passport control in Laos - similar to Chinese trains - similar to entering an airport - nothing too serious.

Cannot comment on train ticket purchase, as I only took the train one way.

We were about 10 min late pulling into Luang Prabang due to some tunnels that were being maintained (cracks being patched up) and the train having to reduce speed.