The train health is part of the map itself. I'm not sure if there is a way to remove it.

It's probably controlled by some textures used in the map, if it's possible to find out which, we can make them transparent.

you need to edit the vtf file for the train,

google Nem's tools VTFEdit

You probably need GFScape to rip it first

VTFEdit isn't gonna do me any good if I don't know what the file is. Just going through the TF2 vpks I got a ton of results when searching for stuff like 'train'. I can't really go through all of those.

I figured out how to do this but it does require sv_cheats to be set to 1 (Shouldn't be a problem since you're recording). I decompiled the map to see how the HUD works and after flipping through of the logic_auto entities I found where stuff was getting called and used ent_fire to disable it.

ent_fire hud* stopoverlays

Putting this in should work but there's a chance that some other stuff could get broken if you manage to trigger any of the entities that set up the HUD.

? theres definitely a folder called materials/snowplow or something to help you narrow your search.

you really can go through all of those.

Well yeah there's a folder called that, but it's packed into the .bsp file. I can even open all the vtfs repsonsible for the health icons, but I can't change them inside the bsp.
I tried to open the map with VIDE to re-pack the materials with the icons made transparent, but VIDE won't even open cp_snowplow for some reason. (http://puu.sh/k9hJY/0f78afedef.png)

And yeah Benroads solution works for when you are currently playing on snowplow. But when watching a demo it sadly doesn't.

So yeah I'm still looking, starting to feel like this train icon was made to stay...

r_screenoverlay 0
Try this command, the zero can be anything it doesn't matter also it resets when it goes down a percent so you'll have to use the command again.