Some notes about plants along the lines ....
Upper Holloway Station (Islington)
Species on the bank behind the eastbound platform included Russian Comfrey, Black Medick, Common Vetch, Geranium dissectum, Geranium robertianum, Spear Thistle and Oxford Ragwort.
There were several Wall Lettuce behind the westbound platform.
A distant (for the magnification) binocular view of some white-flowered Comfrey beyond the publicly accessible part of the westbound platform strengthened my view that it was probably White-flowered or Soft Comfrey (Symphytum orientale). A closer glimpse out of the train window as I headed west confirmed this opinion. It is said to be common in southern and eastern England, but other than my own plants at home, and one seen in Bristol, I don't recall coming across it much. What was likely another specimen was seen just east of Hampstead Heath station.
At Brondesbury Park there was a large amount of Japanese Knotweed behind both platforms. A row of Greater Celandine was just outside Willesden Junction.
There was Hop, Garlic Mustard and Green Alkanet just west of Kew Gardens station.
Between the lines heading east out of Richmond were Laburnum and a Broom.
Several Lilac graced the lineside between North Sheen and Mortlake.