What's the point of opening the train doors in the White X-mas Heist?

Sometimes you can get the gage packs as you said, another time I've seen an ammo bag inside.

Other than that I'm sure you could probably also get a medic bag in there as well. Not sure about presents though but i highly doubt they'd put a present inside a train that can't be accessed if nobody brings C4.

I think this is meant as a defensive position. You can either hide loot bags there or hide there yourself as you wait for the helicopter to come to that part of the map.

On the White X-Mas heist, opening train doors has a high chance of yielding:

• A medkit box, similar to the ones found in the Hoxton Breakout, 2nd day, Infirmary
• An ammo bag, similar to the ammo deployable in the Enforcer tree.
• Gage packages, randomized

Additionally, as others have mentioned, it's good for cover, as opening only one door will force enemies to funnel into one chokepoint.