The location in your photo is the Richmond Hill line at Queen Street. The speed limit there is 35 mph (56 km/h), but it is immediately adjacent to the 25 mph (40 km/h) Permanent Slow Order around the sharp curve at Corktown Common park, so trains would be going less than 40 km/h.
I recently encountered some teenagers playing football on the tracks in Rijswijk station - an underground station where intercity trains pass through at 140 km/h, six times per hour in each direction (plus four local trains per hour which stop). These teenagers were well aware of the risk of approaching trains, as every few minutes when a train whizzed through, they climbed up to the platform and held their ball tight. Standing on the platform in an underground station while a double-decker train whizzes by at 140 km/h is already a heart-pumping experience, which certainly makes one well aware of the danger of being on the wrong side of the platform edge line.
The reason they were playing there, rather than the park directly above the station, is likely because of the deadly danger every few minutes - they think that defying the rules about staying off the tracks makes them seem tough to their friends.
Given that the people in your picture are likely fully aware of the danger of trains, I don't think it would be essential to stop the evening Richmond Hill line service. Slowing to a crawl and applying the horn seems like it would be perfectly adequate in this situation.
If the customer service rep with whom you spoke also didn't feel that it was an urgent life-and-death situation, then they might not have immediately called the emergency number for train dispatch (the one posted at every level crossing), and it may have taken more than 10 minutes for the message to reach the applicable train crew.
