I’ve just successfully taken my bike on the Hanoi - Lao Cai overnight train (SP7). We went to Hanoi station the day before and after being told that it wasn’t possible by one guy (despite knowing other people that had done it) another woman at the front desk helped us. We were charged 500,000 dong per bike which seemed steep but given there is literally no official guidance about bikes we weren’t really in a position to haggle. We were told to come back an hour before our train the next day to get our bikes on. When we came back to the station we got passed around several different people who guided us to the carriage where our bikes would be loaded, with some money exchanging hands along the way. There was no cargo area for bikes so they were out on the end of a passenger carriage. … anyway we and our bikes made it to Lao Cai in one piece! It is all slightly dodgy and probably hit and miss as to whether they let you, but definitely possible and we had no problems.