Chris,
Stan is absolutely right, use either the cheap 12VDC, 2Amp wall-wart or like many of us, use the 18/24 RS power supply with the M adaptor to power the TIU thru the Aux port.
Further if you are expanding your layout and want an economic way to set up, grab an old ZW with the 4 channels, put some 10 Amp Breakers between the old Transformer and the TIU and you have all kinds of options for running your layout.
Have fun building your layout!
PCRR/Dave