The old man did not wish to be recognised. No matter his precautions, there was still a chance of that though. On his way to the train station he stopped to buy a magazine. Something about cars should be a sufficient disguise.
The train arrived one minute late, just as he remembered from the first time he was here so many years ago. He found the seat he needed, near the centre of the carriage, beside the young familiar face. The young man was reading, near the end of a long novel. He would not look up. The appearance of an old man was nothing remarkable, and the magazine would throw off any hint of recognition.
Between two stations, the old man folded his magazine and braced himself against the seat carefully. That was when the train lurched, jumped off the tracks and toppled over, sliding sideways along the tracks in a cacophony of distressed metal and screaming people.
When the train came to a rest, the old man was unconscious. The young man was injured, and felt like just sitting still, but saw the old man lying across the seats, helpless. Through the smoke and the hot metal, the young man managed to hoist the old man up and carry him the short distance away from the tracks, to relative safety. Just as they cleared the carriage, it started burning and filling up with choking smoke. Another few seconds inside and they would have both been dead.
The old man woke, coughing and spluttering, he grabbed the young man by the arm with surprising strength, and spoke: "When the time comes, remember why you did this." Then he passed out again. He never woke up.
The young man sat staring at the old man for a while, trying to place his face. Was he someone he'd seen on the train before? Maybe he was a distant uncle he'd met at some family gathering.
Then it all started coming together. That nose. The mole to the left of his chin. The scar on his right arm where he - they both - had fallen when nine years old. He shook his head, as if trying to dislodge the impossible idea, but there was no denying it. The old man had saved his life because they were the same person. And when the time came, somehow, he would have to return the favour to himself somehow.
Mokalus of Borg
PS - I dug this almost fully formed from my writing ideas file.
PPS - I haven't spent as much time on it as I would have liked, though.