I had to go back to the beginning. I was thinking your problem was how to power the TIU itself via the AUX PWR jack. Now I see this is an HO DC application. OK - now I'm onboard!
Being the consummate cheap-skate I cringe at the cost of the MTH pin adapter! 
For less than $1 you can get a coax plug to screw-terminal adapter...albeit you'd have to wait for Asia shipping. So it would be something like this:
In this example, the DC-output laptop adapter is shown set to 18V DC. The TIU itself will steal about 5-10 Watts of power to do its thing (unless you provide external TIU power via the AUX PWR jack). That leaves ~80 Watts of motive power which is generous for most HO layouts even driving multiple engines.
Note that Joe mentions G-gauge power. This really is a case of apples-oranges. The Bridgewerks supplies are DC beasts (in the positive sense of the word) but overkill for your application.
As for the TIU being "smart enough" to deliver the correct amount of power, it's actually the engines that only draw as much power as they need. Think of it like your house wall outlets. You can plug in a 1 Watt nite-lite or a 1000 Watt hair-dryer. Hope that makes sense.
