Before getting stuck into data on performance, it's worth understanding how our rail services were split into two parts when privatised in the 1990s.
Network Rail handles infrastructure, such as the track, signals and overhead wires across Great Britain (England, Scotland and Wales but not Northern Ireland). Although Network Rail is not a private, shareholder-owned company like its predecessor, Railtrack plc, its status remained ambiguous until 2013 when the Office of National Statistics ruled that it was a government body in the public sector. As a result its debts of £34 billion were added to the UK's public debt.
The train services that run on Network Rail's infrastructure are operated by private companies. They procure and maintain rolling stock, manage staffing and are responsible for many train stations. Trains in Scotland are run by the ScotRail franchise which is awarded to private companies in a competitive tendering process. National Express won the first contract in 1997, and were followed by FirstGroup in 2007 and most recently Abellio was awarded a ten year contract starting on 1 April 2015.
Whereas Network Rail must reinvest any surplus (if it ever made one) in its infrastructure, the private companies could choose to distribute profits (if they can make any) via dividends to shareholders.
Punctuality - long view
The information presented here is mainly from the Office of Rail and Road's (ORR) Passenger and Freight Rail Performance report for 2016 Q2 published in November 2016.
First, take a look at the history of punctuality since 1998.
MAA stands for Moving Annual Average which means an average is taken over the year leading up to the stated date. This smooths out short term variations, emphasising trends and easing comparisons. Information is of course lost in doing this, most notably a 5 to 10 percentage point drop in PPM that occurs every winter (see first graph here).
With this in mind, you can see what the above graph is telling us. Train punctuality fell suddenly in the early 2000s, then had a slow recovery and since 2010 has stabilised at around 90% (except in London and the South East where a decline is evident).
The plummet in PPM in the early 2000s followed the Hatfield derailment in 2000, which killed four people and injured many more. This tragedy prompted an investigation which uncovered serious flaws in track inspection and maintenance that had developed in the years since privatisation (whether it was caused by privatisation is another matter). To prevent many potential repeats of Hatfield, an extensive programme of investment in rail infrastructure was instigated and the disruption caused by these works is why PPM suffered in the early to mid-2000s.
Punctuality - recent
The chart below shows latest PPM MAA figures for Train Operating Companies (TOC) such as ScotRail. These figures are for 2016-17Q2 (Jul to Sep) and the difference from Q2 in the previous year is also shown. Abellio has operated Scotrail during the whole of averaging period for 2016-17Q2, but FirstGroup operated ScotRail for two of the four quarters in the 2015-16Q2 average.
ScotRail sits in the middle of all GB train companies at 89.6% which is a 1.2 percentage point drop on the previous year. This drop is typical of most companies.
The situation is little changed if we look at data for 2016-17Q2 without the averaging. ScotRail's PPM was 90.2% with a drop of 2.0% from 2015-16Q2.
According to data for the latest official period (16 October to 12 November 2016), available from this Network Rail page, PPM was 87.0%, up from 84.3% for the same period in the previous year. PPM MAA for the year to 12 November 2016 was 89.8%.
The same page also gives statistics on who was responsible for any passenger train delays of 3 minutes or more. For the entire GB network in the year to 12 November 2016, 61% of delays were caused by Network Rail, 29% by the train operator itself and 10% by other operators. For ScotRail these figures are 54%, 38% and 9%. Out of the 22 train operators listed, only three others had larger self-caused delay percentages than ScotRail.
The ORR report also gives some European context. GB ranks 9th out of 22 European countries for long distance PPM, but only 18th of 23 European countries for non long distance journeys.
In short, ScotRail's performance has been unremarkable in PPM terms: neither particularly good nor bad relative to rail companies across GB. However, ScotRail ranks near the bottom for self-caused delays.
The numbers presented here do not show that ScotRail has a serious problem. That's not to say it doesn't though. We'll have to wait and see if data beyond 12 November will show what people have reported anecdotally. It is also possible that legitimate concerns have been amplified by both social and traditional media.
Daily PPM data is available, such as from trains.im or uktra.in, and at the time of writing (17.31 on 24-Nov-16) it showed ScotRail as having the fifth worst PPM in GB of 75%. But caution is required over short timescales for a variety of reasons. For example, in the last week it was pointed out that ScotRail's performance was amongst the best in GB, but this was while the southern part of Britain was beset by Storm Angus.
There's one point worth noting here that applies beyond just ScotRail. The most problematic part of our railway system is Network Rail; it is responsible for over half of all delays. But, the only reason train operating companies stand a chance of making a profit is because the most heavily loss making part of the railways has effectively been nationalised in Network Rail.