Train services between Nottingham and London St Pancras were disrupted by a group of horses on the line on Wednesday morning.
East Midlands Trains said the animals were spotted on the line bear Wellingborough, Northamptionshire, on the route between Nottingham and London.
It led to delays of up to 30 minutes on services. Video from one train passenger showed Network Rail staff in high-visibility jackets trying to direct four horses away from the tracks.
The animals were moved away from the line at around 11am and normal journey times resumed shortly afterwards.