Yeah, it is frustrating not to be able to capture the tanks from the incoming train...
I managed to finish that mission by sending 3 guys crawling to the heavy tank, pull back behind the trench and shoot everybody from here. At one point, I got my tank disabled by a lucky shot, but the big part of the job was done. The rest was just raiding the AA guns using grenades (or distract the gun crew with a guy on the other way, and shoot the bastards from behind
)