Stay off the road and the tracks yourself
The hardest instruction in this emergency is aimed at you: do not run after him into traffic or onto railway land. A dog chasing a deer, rabbit or fox is running on instinct and will barely register you, while a pursuing owner adds speed to the chase and puts a person in the path of vehicles or trains. Railway land is especially unforgiving, because trains are fast, quieter than expected and cannot stop for either of you. Your job is to work the problem from a safe position, however wrong that feels in the moment.
Call smart from where you stand
Position yourself where he entered the road or embankment, or at the nearest safe point he is likely to return to, and call in the voice and pattern he knows from training — his name, his recall whistle, the phrase that means food. Keep calling at intervals rather than constantly, and crouch or turn side-on when he glances up, which reads as inviting rather than confrontational. If you can move parallel to his direction on a safe path, do so; many dogs curve back toward the point they last saw their owner, so staying findable matters as much as being loud.
Railway and road emergencies need official help fast
If he is on railway land, phone the emergency services or the railway's emergency line straight away, give the location as precisely as you can using landmarks, bridges or location signs, and state that a dog is on the line. Signallers can caution or stop trains only if they hear in time. For a dog running in a road, warn traffic only from the pavement or verge, never by standing in the carriageway, and call the police if he is creating a danger to drivers. Do not wait to make these calls until you have tried everything yourself; help mobilises slowly, so start it early.
When he comes back — or does not
When he returns, resist the urge to scold: he came back, and punishing the return teaches him to hesitate next time. Clip the lead on calmly, check him over for cuts, thorns, lameness or signs of exhaustion and overheating after a hard chase, and offer water. If he does not reappear quickly, treat it as a lost-dog search from that spot: stay in the area, alert nearby walkers, and get word out while the trail is fresh rather than heading home to organise. Note the exact entry point, because searchers will start there.
Prevention for a dog with a chase drive
A dog who has done this once has told you his recall does not yet beat wildlife, so the management changes: long line in any area with deer, rabbits or livestock, no off-lead freedom near roads or railways even if the field itself looks enclosed, and serious recall training built up around distractions gradually. Choose future off-lead spots for their distance from traffic and tracks, not just their size. And if a chase ever ends with him missing rather than returning, KinsLe's Lost Pet Alert System can alert nearby members around that spot while you hold the area and keep calling.