Rail services were again disrupted by people in Standedge Tunnel at Marsden.
Drivers were ordered to slow down on Sunday afternoon after a man was seen inside the tunnel.
Police are now stepping up patrols at both ends of the rail tunnel, which carries the Manchester to Huddersfield service deep below the Pennines.
And they have repeated an appeal to parents to stress to youngsters that playing near railway lines is dangerous.
Kirklees Neighbourhood Rural policing officers joined colleagues from British Transport Police to investigate’s Sunday incident.
A BTP spokesman said: “We had reports yesterday at 3.15 pm from a train driver of an adult male trespassing in the tunnel at the Marden end.
“Trains were placed on caution through the tunnel. Officers attended and searched the area but no-one was found.
“We have attended a joint action meeting with Network Rail and the train operators about the issue and we have introduced a new response plan to the area for both policing teams at either end of the tunnels, on the Marsden and Diggle sides.
“We have increased patrols in the area and Network Rail are looking at upgrading the gates at both ends.
“We are still keen to reinforce the message about the dangers and for children to stay away, as it is still school holidays for many.”
The first incident happened on Good Friday, when dozens of trains were halted or delayed because youngsters were playing near the busy main line.
Rail companies had to stop many services on the busy Huddersfield to Manchester Line while British Transport Police searched a network of tunnels alongside the railway.
Some eight to 10 youths were seen trespassing in disused tunnels next to the railway line and a full stop had to be put on the line whilst officers searched the extensive tunnels which link to the railway line.
The tunnels are gated but youths gained access.
Take a trip through the tunnel below.