Sentence:  When we see a train trapped behind (or embedded in) a fallen <WSD>tree</WSD> our first thought should be 'what was it doing there anyway?' [...] <WSD>Trees</WSD> are also responsible for numerous minor delays in autumn <small>[due to leaves falling on the track]</small>, which rolling stock engineers are supposed to cope with as usual. 
Meaning of the word marked with <WSD> tags:  A perennial woody plant, not exactly defined, but differentiated from a shrub by its larger size (typically over a few meters in height) or growth habit, usually having a single (or few) main axis or trunk unbranched for some distance above the ground and a head of branches and foliage.