Hi, Me lim. Mutianyu does not have rail connection with Tianjin. You can take train from Tianjin Railway Staiton to Beijing (perhaps Beijing South Railway Station by bullet train). Than take subway Line 4 from beijing South Railway Station to Xuanwumwn station then change to Line 2 to Dongzhimen. Get out to find Dongzhimen bus station nearby and then take tourist bus line 916 or 936 till the end, Huairou. After that, get in a minibus to Mutianyu Great Wall or you can just take a taxi. it may cost 30CNy to take taxi and 2 or 3CNY to take minibus (a kind of light blue minibus).