Office location of TCG

I plan to take train to Ulan Bator(MNG) from Beijing in July, 2015. I will stay in Beijing for about two weeks before leaving for Ulan Bator(MNG). According to the info, we (2 persons), can buy ticket at least 7 days ahead (or mail/fax you our visa/passport months before for reservation), but where is your office location in Beijing? Tnx.