Updated: 5 Mar 2003

Left: The only known photograph of Fowler's Ghost, a locomotive intended to run through the tunnels of London's Metropolitan Railway without emissions, using heat stored in firebricks. 'Fowler's Ghost' was an unusual- and disastrously unsuccessful- locomotive built to run on the Metropolitan Railway in London. This line was and is just below street level, much of it being built by "cut and cover", but some of it is in open air. There was concern about steam and smoke emissions from the locomotives, and Sir John Fowler, the Metropolitan's Engineer, ordered an experimental locomotive designed to not produce smoke while in the tunnel sections. It was built by Robert Stephenson & Co of Newcastle, arrived in 1861, was tried on a few test runs, and was never seen again...

The experimental locomotive remained a mystery for many years, and apparently received a first-class cover-up; hence the name Ghost. Allegedly it was referred to in an address to the Institute of Civil Engineers in 1885; really must try and look this up.

Little is known of the 'Ghost' on trial, but a disastrous foray down the Great Western line, about 7.5 miles to Hanwell station is recorded; it appears things went badly wrong and there was much difficulty in getting it home; another attempt, on home territory between King's Cross and Edgware Road, is also on record, but seems to have gone no better.

Left: The internals of Fowler's Ghost. The Ghost' was a 2-4-0 tender engine. The gauge was that of the Great Western, 7ft 1/4 inch. There was a small firebox, a very short length of tubed boiler over the leading wheels, and a huge combustion chamber between the two, where the firebricks lived. When running in the open track these bricks were heated by the combustion gases, while any spare heat raised steam in the boiler. In tunnels, dampers were to be tightly closed, to prevent emission of smoke, while the fire-bricks kept the boiler working.

The choice of fire-brick as a heat storage medium was a bad one. It would have been far better to use a normal boiler, as water, having the highest specific heat of any substance (for example, metallic mercury is only 0.033, which is rather surprising) would store heat much better. A big well-lagged boiler should have done the trick.
Alternatively the fire could have been done away with altogether, and the boiler topped up with steam as required. Plenty of fireless locos have been built on this principle, but whether they could have had the range and power to work the Metropolitan is questionable.
(Update: I have recently learnt that a fireless loco was tried before the Ghost was built, but was indeed deficient in both power and range)

Ultimately both the Metropolitan and the District were worked with what might be called semi-condensing tank locomotives: these produced smoke in the normal manner, but their exhaust steam, while running in tunnels, was directed via a valve back into the water tanks and, partly at least, condensed. Since the tanks quickly got hotter and hotter the condensation became less and less effective, and the tanks had to be drained and refilled with cold water at the end of each run.

It should be said that Sir John Fowler was not an incompetent nitwit; he went on to co-design the Forth Bridge, which seems to be holding up nicely. Just goes to show that even engineers are fallible.

C. Hamilton Ellis famously summed it all up: "the trouble was that her boiler not only refrained from producing smoke, it produced very little steam either".