Dr. William R. Hanson writes in The Adventure of the Detective Stamp:
Two locomotives named "Sherlock Holmes" have rolled on British rails, and at least one has been depicted on a stamp. For the paired 60-cent values of the 1985 Funafuti-Tuvalu "Leaders of the World Trains" series (Scott 32), the 1920 London Transport Diesel was shown as side and end elevations on one stamp and in action on the other.
[It seems this should be corrected to "1984, Scott #20"]
A 7¼ inch gauge Metropolitan diesel-electric locomotive 'Sherlock Holmes', 4588, battery-powered with panel coachwork, handrails, buffers and lights, finished in lined red and brick lined livery, 160cm long, the cab 35cm wide; together with a driving trolley, with fitted speed control.
[auction sale listing. Sept. 17, 2013: sold for £2,728 (including the buyer's premium)]
(thanks to Peter Blau for sending this)
Apparently, after two or three years, the (1920) diesel-powered locomotive (on the stamp) was replaced with an electric-powered "Sherlock Holmes":
This information was presented at MetroLand:
The Metropolitan Railway (MET) operated over the northern side of the Inner Circle from Aldgate where it ran trains to Aylesbury via Baker Street, Harrow-On-The-Hill and Rickmansworth...
... The MET had also taken delivery in 1922/23 of twenty new electric locomotives to replace the earlier locos which were underpowered for the longer haul to Rickmansworth. The new locos were also Bo-Bo arrangement but had slimline bodies with a blunt V shaped cab end. All of them were named after prominent historical figures with connection to the locality served by th MET, Sherlock Holmes and Sir Ralph Verney. These locos hauled the Aylesbury trains of bogie stock from Aldgate as far as Rickmansworth where they were changed for steam traction for the remainder of the journey, one of the features of this service was the highly efficient changeover of locomotives that usually took as little as three minutes to achieve! Locomotive haulage ceased in 1961.
At the Duxbury Leamington & Warwick Model Railway Society site appears this photo of the Metropolitan Railway Electric Loco No. 8 "Sherlock Holmes," built by Steve Johnson, to appear on the Duxbury tracks:
These references from the University of Minnesota's Universal Sherlock Holmes page may provide additional information on the locomotives:
Railroad Locomotive -- Sherlock Holmes
C17531. -- A4859. Day, John R. "S. H. Locomotive," Sherlockian Who's Who and What's What. Edited by W. T. Rabe. Ferndale, Mich.: Old Soldiers of Baker Street, 1962. p. 2-6.
A brief history and description, with three photographs, of the No. 8 locomotive named after Sherlock Holmes.
C17532. -- A4860. Prestige, Colin. ["Letter"], The Baker Street Journal (New Series) [BSJ], 4, No. 2 (April 1954), 122-123. (From the Editor's Commonplace Book)
A description of the ceremonies attendant upon naming the railway engine of Great Britain's Metropolitan Line "Sherlock Holmes."
C17533. -- A4861. "The Sherlock Holmes Locomotive," The Sherlock Holmes Journal [SHJ], 1, No. 4 (December 1953), 29; 6, No. 2 (Spring 1963), 60.
Scott # 1, 4, 8, 10, 13, 18, 20, 22 - 12/24/1984