All stations trains around 6pm

Your answer lies in many factors beginning with the fact that the Metropolitan has one of the most complicated service patterns on the Underground due to a number of factors some being historical and others reflecting the political climate that influences TfL from City Hall.


The Met can be seen more as a suburban railway than a tube line with it's services stretching well beyond the Greater London boundary with Fast, Semi-Fast and All Stations services running via four branches all funneling down one trunk route that itself amalgamates with other lines in zone 1.

Since the introduction of the S stock, the service pattern has gotten more complex with Fast and Semi-Fast services running Southbound only in the morning peaks and Northbound only in the evening peaks (leaving the contraflows out of this one).

This in itself creates the problem of a lopsided service with train services being accelerated in one direction but not being compensated for in the other requiring some handy scheduling. 

Then there is the question of demand which on newer lines is easier to meet due to fewer branches, more reversing points/sidings/platforms and isolated/independent infrastructure such as that found on the Victoria line. Due to the Mets complex history, it grew out of a messy tangle of private companies that were merged and amalgamated as they went along. This has left the Met with several issues regarding capacity because lines south of Baker St compete for space with Circle and H'Smith and City line services and Baker Street has two deadened/reversible platforms that are already used intensively in the peaks. The next reversal platforms are at Moorgate but any increase on Met services here would encroach on the Circle and H&C which arguably have just as much cause for more capacity.

With so much intertwining with other lines any disruption on the Circle or Hammersmith and City by just a couple of minutes can leave large gaps on the Met lines service with trains stacking up. Service controllers may often divert trains to run via the fast lines to make up time. This practise is something only possible on the Met with it's four track infrastructure largely a hangover from it's days of competing with mainline railways in decades gone by. Beside from the peaks, the fast lines between Wembley and Harrow see little use as the current service has been simplified greatly over the years. The irregularities you experienced on your way to Preston Rd during the peak show the issues that the Met faces with meeting peak demand in the suburbs whilst balancing that with the borders of Buckinghamshire and Hertfordshire running along side the Circle, Hammersmith & City and Piccadilly lines aswell as Chiltern Railways whom have their own service fluctuations.

Any timetable change how ever minor can be amplified by the quirks of a line that has historically and still is by many viewed as an oddity.

Not referring specifically to the Met but the fact that in the early years, all of the lines were in effect private enterprises trying to out do each other. The Mets main competitor in those days was the Metropolitan District Railway and the Great Central Railway. What I'm getting at is that if the line were to be built in it's current form today it would raise a myriad of questions and doubts over the practicalities of this purple giant. 

During the morning, taking a southbound train, I regularly notice semi-fast services going northbound (usually going Harrow) so probably some service recovery. Unless I am reading the working timetable incorrectly however, I do see some trains pencilled in to Watford as semi-fast.