Tornado is the first of a series of 'new build' locomotives as opposed to 'rebuilt' (from previously (sold for) scrap) locomotives. New build I believe have different regulations to rebuilds as would be expected as the basic chassis, though stringently inspected (10 year limits normally for a full inspection) are still old. The Tornado trust hopes to have a higher speed limit tfor its locomotive than the rebuilds: 95 mph is suggested, the rebuilds speed limit is 75mph for main line use. Preserved railways are different - they have much lower speed limits.
New builds take a long time and are very expensive, heavy engineering is never cheap.
One of the new builds is of a London Midland & Scottish Railway Patriot class 4 6 0 which will, when finished, be a ‘war remembrance’ loco, so far they have the frames cut and are being assembled. The association doing this have a web site at the following location http://www.lms-patriot.org.uk/overview.html
Another is a passenger tank engine, I do not profess to know all though I am sure there are others in the pipeline.
Steam for (normal ) main line use NO, it can not fit into the present speeds that either electric or diesel haulage can produce. It is also becoming apparent that the paths for the steam specials are getting harder to find – there is not enough available, not helped by large sections on some line which produce a limit on the number of paths available; the railways are separated into track & signals/passenger franchise & freight franchises + enthusiasts/steam train/dining specials. Things get tight!
Equally steam engines are filthy things made more so by the new regulations that require a full sheet over the gap between loco cab & tender. That is needed to keep high voltage electric power (25 kilovolt) away from the loco crew.
The loco is as you say an updated steam loco but performs just like the others of its class that were tested, and are now scrapped – it is fast, and performs well. Specially trained crews are needed, and there are not a lot of those – they seem to do the at times difficult, and hard job for the pleasure of doing it, but they are paid I am sure.
More locos are being suggested – the really difficult bit is the raising of the large amounts of money first to build them: they are built mostly with cash raised by volunteers. Tornado has a large loan which is being re-paid partly with fees from the trains that it runs.
Don’t forget we have no large steam era locomotive works left, though there are small ones that do a very good job indeed! Needless to say they are busy.