[ROSOBORONEXPORTCORP]

I would expect a locomotive to be a 'little' stronger than a carriage....in the default setting they are all the same...

Definitely, although one shouldn't underestimate the durability of underlying structure on most railcars (especially heavy freight cars), most engines were indeed built like tanks.

Half an inch isn't that thick. I have many .30 cal rifles, and I've put holes in 1/2 inch plate with AP rounds. A .50 cal BMG API will easily penetrate 1/2" at a several hundred yards, IMO.

Here's a pic I found:
http://img59.exs.cx/img59/919/ap19az.jpg

Looks like .50 cal will penetrate over 1/2 an inch at 600 yards, a little shorter range if it's at an angle (not a problem with a cylindrical locomotive , some bulltes will almost always be normal tothe surface) )


tater

Tater, lets switch to metric:
According to the chart in the image you posted, at 600 yards, a .50 cal direct impact on a steel plate (uncurved and unstressed) will produce at most 16mm penetration. This rises to as much as 20mm at close range.

Typical boiler thickness (if you examine my post) is estimated at around 110-120mm. This means that the penetration of the Browning 0.5 is five to six times too low to make it through the boiler (even at a 'perfect' impact angle).