I want to add led light to the front (and probably back) of the Red Cargo Train but the shape of the light is making me wonder what brick I should use to hold the light. I've never seen these lights used anywhere. I'm thinking I'll need something from a Technik set.
These light fit in a regular Technic hole, and thus have the same diameter as a stud.
I would recommend fitting them in the so-called "Erling" brick; after all, one of its regular (aka "boring") names is "Brick, 1 x 1 with headlight":
The only problem you'll have is that the light extends quite a bit at the back, but you can easily solve that by using a panel of some sort, for example:
I used a grey 4 stud Technic brick with 4 holes because didn't have two grey 1 stud 1hole Technic bricks. The LED are about 1.33 units high so I had to raise the black platform height to accommodate the width of the LEDs. When I added lights to the Train Engine Operator cabin - I had to create more room inside so the train engine became a bit less sturdy. You can see photos and a video of my solution below.