Hello, Ms Ng, ther is some info I know. First, you can buy the ticket from Beijing to Datong at Tianjin Railway Station. Tianjin Station has this service to sell ticket of other places. Second, if you but the ticket from Tianjin to Datong, you can get on the trian in Beijing, because Beijing is within the route. However, the point is if you buy sleeper ticket from Tianjin and no body is there on the bed, the ticket will be sold again one hour later. That is to say, you ticket maybe invalid any more. So, it is better to buy Beijing West-Datong ticket at Tianjin Train Station. Make all these clear by enquiring the ticket office. :-)