The `` M-Stock '' ( or CM-Stock '' ) of `` M-Train '' are the oldest trains on the MTR , built originally by Metro Cammell ( now Alstom ) and refurbished by United Goninan . The M-Stock ( or CM-Stock '' ) of M-Train are the oldest trains on the MTR , built originally by Metro Cammell ( now Alstom ) and refurbished by United Goninan .