I am currently taking a holiday in North Wales, and have taken the opportunity to gather some pricing information in respect of some of the narrow gauge lines in the area, which is potentially useful now that Pak128.Britain has some narrow gauge items (based on the Ffestiniog Railway, it seems). I have not always had time to note the names of the books from which, by browsing them in bookshops, I have garered these data.
A narrow gauge slate wagon in 1901 was recorded as costing £59, and a passenger brake in the same year £79. I am told by staff at the Welsh Highland Heritage Railway that the normal capacity of slate wagons was 2t on the narrow gauge, heavier wagons tending to derail (although there seem to have been larger wagons for coal and goods from about the 1920s onwards).
The Veil of Rheidol's locomotive "Rheidol" 2-4-0T was purchased secondhand for £591 in about 1902, having previously been owned by contractors constructing the line. A larger 2-6-2T of the same line (no.9) is recorded as having been purchased for £1,750 in 1902 and a seemingly identical locomotive (but query whether by then fitted with suprheating) in 1923 for £2,737 (no. 7).
A small four wheel "toastrack" carriage (no. 42) built for the Welsh Highland Railway in 1923 (and preserved by the Welsh Highland Heritage Railway at Porthmadog) cost £155 new. It was open sided, had hard slatted wooden bench seats, and would not have been very comfortable.
A batch of 4 corridor bogie coaches built in 1923 was recorded as costing £2,604, and a batch of 2 good brakes in 1902 were recorded as costing £168 (both for the North Wales Narrow Gauge line).
The Ffestiniog Railway's second steam locomotive (to the same design as its first and a number of subsequent locomotives), known as "Prince", cost £975-7-6 when new in 1863, and had a tractive effort of 4,489lbf.
A "single Fairlie" locomotive for that same line built in 1872, and considered at the time to be faster and more economical to run than other locomotives on the line, including the more powerful "double Fairlies" (the design of the current narrow gauge steam locomotive in pak128.Britain), cost £1,305 new.
A "double Fairlie", by contrast, when built in 1869, cost £1,905, although the source that I found expressed some equivocation as to the price, recording that it might have been as low as about £1,600 or so.It weighed 24t and had a tractive effort of 6,059lbf.
The North Wales Narrow Gauge (later Welsh Highland Railway)'s now preserved "Russell" of 1906 weighed 20t and had a tractive effort of 7,425lbf, although I cannot find pricing information for this one.
Deviating briefly to Ffestiniog liveries, two basic schemes for locomotives seem to have been employed (with some minor variations that would not show up on the Simutrans scale), with the original 1863 scheme being red with black lining and a new scheme adopted in 1925 and initially continued into preservation being dark green with yellow lining (the original livery for locomotives, or near enough, having been restored somewhere around the1990s, I think).
Returning to prices, the Ffestinog carriages nos. 21-22, bogie compartment carriages built in 1897, cost £305-12-5 and £308-1-4 respectively when new.
I should also note that the Ffestiniog Railway's own online encyclopedia has some very good pricing information.