Good spots for sentry guns:
The train yard is very open, and hence difficult to defend well with sentry guns. However placing one behind the flatbed carriage in the middle can be a good position to defend against attackers entering the train yard.
A more temporary but perhaps more useful position for a sentry gun would be on the top floor of the train yard, providing support to those members of your team capturing the point. Once the point is taken, consider moving it to the enemy’s side, especially in the little balcony part.
Outside of the middle train yard, the area out the front of both teams’ bases can be defended by a sentry gun on the wooden planks behind the flatbed carriage. It is able to shoot over the top, but has at least some cover for its legs.
More useful as a forward base, sentry guns can built by attackers in this area out the front of the roller doors, either behind the flatbed or in the underwater intersection.
When pushed back inside to defend the second control point, placing sentry guns on the wooden second floor allows them to fire down on attackers while presenting very little surface area to make themselves a small target.
The final control point can be defended partially by sentry guns on the top floor overlooking the control point. These cause a degree of resistance, but are quite vulnerable to soldiers.
A better position on the higher second floor is on top of the control point itself, overlooking all the entrances. Once again though, this is quite vulnerable to long range weapons.
A good all-round position for sentry guns is on the wall between both ground floor entrances, forcing attackers to round the corner and expose themselves to get a clear shot. Good defensive pressure should keep these guns fairly safe.