Hi Matt,
You don't actually need a mesh object tied to the light blueprint - it's usually just to make the light child object visible and grab-able in the editor.
Instead of using a point light, which radiates light in all directions and therefore has a greater impact on game performance, even with shadow casting turned off and a PC with some nice kit, I would recommend creating a spot light instead. In the blueprint editor, the blueprint you're looking for when adding a new item is "Spot light blueprint". Give it a name at the top of the blueprint, and you needn't bother filling out the display name fields unless you want to be able to place the light in the route/scenario editor, so I would set the category to "Exclude from browser list". Skip past the Render Component tab and jump straight to the Light Component cab. The radius is simply the distance in metres from the spot light that the emitted light reaches. Phi and Theta take a bit more getting your head around, but then it's easy to understand what they're doing once you've got it.
If you think of a point light, which casts in all directions, as a pie, and the radius of the pie is the value set by the field Radius, then a spot light with the same radius is basically a slice of that pie. Phi is the angle in degrees between the two cuts to form the slice, so if you set Phi to 90, you'd get a quarter pie slice. If you set it to 180, the slice would be half of the pie, and so on and so forth. Theta is a secondary angle that is either equal to or smaller than Phi, and is used to create a light fade gradient between Theta and Phi. So for example, if Theta is the same as Phi, you'd get a hard edge to the sides of the beam of light. If Theta was half of Phi, then the portion of the light beam inside of the angle Theta would be full intensity, and then fade out to nothing towards Phi, giving a soft edge to the light beam.
Edit 07/07/16: Some of the info about the angles above is incorrect, see my later post below. However, the principle still holds.The Colour field is pretty straight forward, and according to Derek Siddle, alpha wasn't used back in TS2012 - I don't know if it matters now, though. It might be worth setting to 1, just in case. Tick Cast Shadows if you want shadows, which you probably do for a cab light designed to be viewed from the outside. Make sure Day night cycle is unticked, and you can leave Anim and Projected texture fields blank.
Once you've created the light object and exported it, you can just add it to the deltics like any old child object, and move it into position. However, unlike particle blueprints, which let you determine the direction of emission to save you have to rotate the particle emitters themselves, you can only rotate spot light child objects to change the light direction unfortunately. By default, they point straight along the positive z-axis of the loco they're added to (length-ways, towards the front), so you need to rotate it 90 degrees around the x-axis to point down at the ground , so that you can stick it up into the cab ceiling (useful link:
http://forums.uktrainsim.com/viewtopic. ... n#p1549720).
Since you want the light to cover as much of the cab as possible, I would probably set Phi to 180, and then you can play with the radius/range and Theta values to find something you like.
Chris