seat61 is a world-wide train-promoting site-somehow it seems to me that many Brits are so fond of those iron roosters they consider any possible negative (whcih there must be-it would be unreal if there were not) as life-threatening assault.
canbypublications is a general Khmer adverstisng site, but somehow is very good on transport/ation.
the best site for an uptodate review of how to go overland TH->KH is talesofasia.com, run by a US guesthse owner in SR, married to a Thai wife.
As per 1/7 coming, there will be direct buslines TH-KH, untill now you always have to change and go by foor over border.
It used to be-as quoted, that 20US$ was the general KH tip for shorttime. As seems to be usual for any tip-advice, there are always people who make a fuss about it and say its too low, etc. A lot of last yrs reports said it was 30. A working boy was quoted, who exlaliend that boss had told them to accept anything from 20 and up without fuss. In the distant past there was a kind of night-curfew in KH, so boys would be quite happy to stay overnight, but that has definitely changed.
DO read up about the habits of Khmer boys-here are now also on this site various reports.