A US dad hopped on a train and rode it half an hour to work before making the realisation every parent dreads – he’d left his baby daughter locked in the car.

The father of two had nearly reached his office in Cambridge, Massachusetts, on Friday, before a feeling of something being wrong started to niggle at him, the Boston Herald reported.

He told the newspaper that he felt relieved that morning that the day care drop-off run seemed to be getting easier. Then he realised why it seemed so easy that morning, he had not done it.

His one-year-old was still in his car, which was parked at the train station.

“My heart sank,” the college administrator told the paper, but said he would not give his name.

He quickly jumped on a train back to his car, ringing the police en route to confess his mishap. Passengers did their best to calm him down.

“It’s a very helpless feeling,” he said, apparently he tried to console himself by concentrating on the fact that it was not too hot or too cold outside.

Police reached the vehicle before he did and rang him to say that his daughter was safe. His wife had also been informed and had set off from work.

When he finally reached the car, paramedics had got inside and were playing with his daughter who was safe and happy.

He told the paper that he takes his two children to school and day care every day. On that particular day, he had dropped his eldest off at school but after was distracted by reports on the radio of a recent fatal Amtrak crash.