Old laptop power adapter to power TIU

That’s fine but you’ll also need to ensure the barrel jack is the correct size 

That's very similar to what I use. The barrel on most of the older laptop connectors fit good. The newer laptop power supplies use a lager connector with a small pin in the middle.

Barrel jack is the standard 5.5mm OD used on older laptops. That's the same size as the TIU aux port correct?

The TIU requires the bog-standard 5.5/2.1mm barrel plug.  Very common, and you can lop off any non-conforming connector and install one for peanuts.  The 5.5/2.5 one will usually work as well, though you may experience issues with the larger center conductor hole making good contact.

Lou1985 posted:

...Since the aux power port and TIU inputs are not connected I should be fine powering the TIU with the old laptop adapter, even though it puts out DC.

Technically speaking, they are electrically connected.  The configuration works though because the adapter and train transformer are electrically (i.e., galvanically) isolated.

gunrunnerjohn posted:

The TIU requires the bog-standard 5.5/2.1mm barrel plug.  Very common, and you can lop off any non-conforming connector and install one for peanuts.  The 5.5/2.5 one will usually work as well, though you may experience issues with the larger center conductor hole making good contact.

Using a 3/32 inch drill bit (3/32 inch = 2.3813 mm) I checked the center of the barrel plug on the laptop power supply. The drill bit fit so it has a 5.5 mm OD, 2.5 mm ID plug. I have another electronic power supply with a 5.5 mm OD barrel plug. I checked that one with the 3/32 inch drill bit and the bit would not fit in the hole, so that power supply has a 2.1 mm ID. I plan on using this power supply, since it has the proper 2.1 mm ID barrel plug. It's rated at 12 volts, 2 amps DC output, which should be plenty for the TIU.

Should work fine, I've powered mine with a similar supply.

Excellent. I'll report back how it works in a few days when I get everything hooked up. 

As an update I used the 12v, 2 amp DC power supply to power my TIU. Works great.

Right.  A 12V 2A is 24 Watts (Watts = Volts x Amps).  A TIU by itself needs less than 10 Watts.   Your original 65 Watt laptop adapter would also have been up to the task.

If you don't already have one, a 12V 2A adapter (with the correct barrel plug for the AUX TIU INPUT) goes for about $2 shipped on eBay: