Would it be possible to make a turret train car and install turrets onto the train car?

Unfortunately, no. Trains only accept artillery type weapons. It could be done by giving a wagon equipment grid, but only Thermal Lasers could be used that way, as equipment weapons can't be fed with ammo in current Factorio version, only with energy.

that's kind of what I was thinking of :) having a train car that has a actual turret grafix on it would be awesome too even if it is just lasers

In factorio 0.16 the heavy tank mod does have an equipment that requires the insertion of fuel into it - I imagine doing the same with ammunition would not be hard.
On the technical side, you could always set the input energy limit to 0 and only add energy to the equipment when consuming bullets from the inventory (or an integrated slot) via a control.lua script.

The animation side might be harder to implement - does factorio allow a post-data phase modification of sprites? If not, you'd probably need a ghost entity that is position synced to the train and renders above it for every possible turret, and create or destroy them in equipment change events.

There is a mod that's similar to this concept. Could probably serve as a point of inspiration/integration. https://mods.factorio.com/mod/Armored-train