I have just noticed the way the Sutton loop is portrayed in Thameslink's
maps.
http://www.thameslinkrailway.com/you.../planning/map/

Firstly, I'm not convinced that the residents of Mitcham Eastfields and
Tooting need to be informed before they even get in the station whether
their train's ultimate destination will be St Albans or Luton, so having
the purple and yellow as separate lines is of marginal value. However,
if they are using the purple and yellow colours in destination blinds of
southbound trains (I don't know if they are), then having the purple and
yellow on the map does serve a purpose.

But the way they have done the loop is so confusing. Suppose you live in
Luton and you work in Mitcham Eastfields, and your wife works in
Tooting. Your wife has a direct train morning and evening, and you have
to change morning and evening. So the northbound and southbound
situation is actually symmetrical, but the presence of arrows on the
loop tricks you into thinking there is asymmetry, and it takes a while
of staring at the map, reading the key and scratching your head to
realise the arrows are a red herring and what the true symmetry is.

What they should have done is just have a single coloured loop like the
Central Line at Hainault. After all, westbound trains at Wanstead
usually go to Ealing Broadway and westbound trains at Snaresbrook
usually go to West Ruislip, but London Underground correctly does not
muddy the map by telling passengers in East London where their train
will go in West London, when so few passenger do these journeys.

But if Thameslink are determined to show the Luton couple that they have
through trains to and from Tooting but have to change to get to and from
Mitcham Eastfields, they should have a yellow two-way line down the
Tooting side of the loop and a purple two-way line down the Mitcham
Junction side of the loop, with the purple blending into the yellow
somewhere near Sutton Common. The software to blend line colours is
readily available, because it's used west of the river in this rather
splendid map of Antwerp.
https://static.delijn.be/Images/RF17_tcm3-5282.pdf