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BBC reports that Gloucestershire Police are trialling a prototype speed camera which has a super lens that can identify a speeding motorist up to 1 km away so if any motorists reading this is in the Gloucestershire Police patrolled area you have been warned, it'll be too late to slow down when you see the camera.

How many long, straight roads are there outside of the motorway network?

Enough to make the investment worthwhile, The Keynsham By-Pass, The A38 south of Bristol Airport, The A37 south of Shepton Mallet, The Bristol Ring Road has some very straight parts to name just a few round here but maybe not on the winding country lanes of Wiltshire, Dorset, Devon and Somerset, Cornwalls a different world.

In my experience, the more twisting a road is, the worse the incidence of dangerous driving on the occasional straight stretches. On a main road which I travel regularly, there are a couple of notorious stretches where motorists, having contained their aggressive and selfish impulses for a few miles, regard it as a point of honour to see how many vehicles they can scream past, notwithstanding hash markings and side turnings, before they slam on the brakes just before the approaching bend.

In Gloucestershire - the Fosse Way (A429) is long and straight for one. 

I suspect the cameras work on line of sight - so if it is a twisting road and you can see over the hedges etc you could pick out a car that is within 1km as the crow flies but actually further away in terms of road distance.

I can remember a talk from the police many years ago when the pro-laser equipment was first introduced and the distance over which this could read.  The comment was made that by the time you see them they have probably caught you

Train drivers who significantly exceed the speed limit, without some truly exceptional mitigating circumstance are liable to dismissal and may be prosecuted.

Car drivers usually get away with it, and if caught face a modest fine.

And whilst it can be argued that a speeding train driver risks many more lives than a speeding car driver, it is decades since even a single train passenger has been killed by a speeding train driver.
Speeding car drivers kill hundreds every year.

Likewise, a train driver who passes a signal at danger will, in the absence of mitigating circumstances, be disciplined and possibly dismissed.
Car drivers pass red traffic signals all the time, facing at most a fine.

And as for drunken driving ! The last fatal rail accident caused by the driver drinking was IIRC over 50 years ago (Hither green) Happens all the time on the roads.

EDIT TO CORRECT the accident of which I was thinking was at Not at Hither green, see subsequent posts for details.
I was also referring only to the UK, accidents overseas are regrettably frequent in some places.

Speeding tram drivers?

Indeed, though it WAS a tram and not a train.
One of the advantages of trams over trains is lower costs, partly by not requiring all the safety features that are applicable to railways.

Hither Green in 1967 was a broken rail. Eltham Well Hall in 1972 was a drunk train driver.

There have of course been incidents since then where a driver was over the prescribed limit. With only luck or location preventing serious injury or death.

https://www.dailyrecord.co.uk/news/scottish-news/boozed-up-train-driver-caught-9032907

http://news.bbc.co.uk/2/hi/uk_news/scotland/glasgow_and_west/8528842.stm

https://metro.co.uk/2014/04/16/tube-driver-was-drunk-on-vodka-at-controls-london-underground-worker-in-court-4701606/

https://www.kentonline.co.uk/maidstone/news/drunk-train-driver-a43801/

So, whilst drink drivers on the roads are statistically more likely to kill, there are still too many incidences of train drivers driving whilst under the influence. And like road drink driving we only hear of the incidents that are reported. No inference should be drawn from the fact there have been no fatalities on the UK rail network, attributed to a drunk train driver, for many years.

A couple of weeks ago, at Totnes, there was a small team of people at the Plymouth end of the Up platform with speed monitoring equipment.

Broadgage- have you forgotten this-?

https://en.wikipedia.org/wiki/Santiago_de_Compostela_derailment

Presumably modern trains are fitted with "black box" data recorders. Does anyone know if these are checked on a regular basis or only after some incident has been reported?

I was referring only to the UK, standards may be different overseas.

On a regular enough basis that most drivers won’t take the risk of trying to get away without reporting something, as they know the consequences of not reporting an incident and then getting found out are far worse.  Newer trains can actually ‘self report’ instances of things like TPWS activations to the relevant control office.