I was thinking the same thing…..the Mallard wouldn't have been far behind it at all and apparently from the text below (that I scabbed off the internet) it did it with a total weight of 240 tons (admittedly on a slight decline):
The record run of the "Mallard" on July 3rd, 1938 was made with a six car streamline set plus a dynamometer car, with a total tare of 240 tons. The Mallard was chosen because it was one of the four engines with Kylchap exhaust at that time. These engines had freer running qualities and higher speed than standard A4. I believe that the Mallard was the first A4 fitted with Kylchap exhaust and had had about three months of service at the time of the speed run. The run started from Barkston triangle, running southbound. The Mallard pulled the train over Stoke Summit at 75 mph, then accelerated downgrade at a gradient of 1:178 to 1:200 over six miles distance to attain a speed of 114 mph. It eventually reached a speed of 125 mph, with a peak at 126 mph for a few seconds. After that it ran at a speed of at least 120 mph for another three miles. By then the inside big end showed sign of overheating and the engine had to run light back to Doncaster for repair.
A German steam train apparently got within 1/2 mph of The Mallard on level ground, so they claim the record should really be theirs.
Does make you wonder if they were expecting it to go a fair bit quicker.
Just found this from their (the Inspiration teams) website:
The late entrepreneur William Lear ventured into the arena of steam powered vehicles during a 1969 program sponsored by the state of California that intended to bring about ecologically friendly steam powered city busses. For this application Lear turned to Barber-Nichols Engineering to design a steam turbine and although the city bus program ultimately failed, several turbines were built in support of the program.
In 1985 one of the Lear turbines found its way into a car which had been brought to the Bonneville Salt Flats of Utah, USA for an attempt at the steam powered Land Speed record. Over the course of several years the car progressed from 110 MPH to a measured speed of 145.607 MPH.
The official speed set by Bob Barber stands at 145.607 MPH but is not recognized by the FIA. However, we at the British Steam Car Challenge recognize this speed as the record to exceed.
And this suggests it should have done 170mph:
Vehicle Details
Length 7.663 m
Width 1.700 m
Tall 1.700 m
Weight 3 tons
Engine Two stage turbine
13,000rpm max turbine revs
Transmission Rear wheel drive
Horsepower 268 kw 360 hp
Top speed 274 kph 170 mph
Fuel LPG Liquid Petroleum Gas
Burners 3 Megawatts of heat
~1500 kettles
~23 cups of tea per second
Boilers 12
Over 3km of tubing
Superheated
Steam Flow rate 40 litres per minute
Temperature 400°C
Pressure 4000kN/m-2 40 bar
Chassis Steel space frame
Steering Rack and pinion to front wheels
Body Front section carbon composite
Rear section aluminum panels
Front tyres Goodyear Speed Eagles 23 X 15
Rear tyres Goodyear Speed Eagles 28 X 15
Water capacity 140 litres distilled water
1,000 litres (1ton) of water used every 25 minutes
Gas capacity 60 litres
Air system 30,000kN/m-2 300 bar
Batteries 4 X 90Ah batteries
Oh and there's a bit of technical stuff here:
http://news.bbc.co.uk/1/hi/sci/tech/4076811.stm