London Commuter ?

Why is there in many places on the side of the track a sign with a flame inside, what does it mean or it there for ?
The most noticeable one, is on the complete left-hand side of East Croydon, going to Brighton

 

"Beware of arsonists"

 

Its a leaf sign. This tells drivers that it is common for leaf fall in the area, when leaves are on the line it can affect the trains adhesion such as breaking etc.. as leaves can cause wheel spin, in reality

 

there are that many in the London area

 

that make sense, leafs can be very slippery, even walking on them, for a few hundreds pound machine it could hazzardous