[From here...]"Keep your eyes out for reinforcements," Steve tells him. Bucky can't help but grin as relief rushes through him like a wave, lifting him up on a surge of hope. "You got it, Cap."
Bucky turns to make his way toward the back of the car to check the door there in case there's more trouble coming, and maybe then to call up reassurance to Gabe, who's got to be worried as hell by now. By the time he hears the mosquito-like whine that indicates the cube weapon powering up, it's already too late.
Steve yells a warning and Bucky drops into a crouch, everything suddenly, strangely clear in his racing thoughts.
He attacks from the doorway, Sharon had said.
It's a little unclear... Steve reported being hit by a blast--He sees it when the blast hits Steve, sees his best friend crash into the side of the train and fall limply to the floor, and sees the shield as it goes skidding across metal. Bucky dives forward and takes up the shield as easily as if Steve had tossed it to him in the first place. He hears the whine of HYDRA's hellish weapon powering up again as he comes up on his feet, and as he instinctively draws his pistol, he can hear Sharon's strained words as clearly as if she's standing there beside him.
--he went on to report several shots fired, most likely from your sidearm, before the cube weapon discharged again...the shot knocks you off your feet and blows a hole in the side of the train--There's no time for thought, no time for anything but action born of the instinctive, desperate need to protect. Bucky drops his gun as though it's burned him and whirls into a pitch instead, using all his strength to sling the shield at HYDRA's agent in the doorway with as much force as he can manage.
It hits him in the chest, and both shield and agent go rocketing backwards into the other car. "Steve, stay down!" he screams, as he runs forward to make sure the goon's out of the fight.