Railway enthusiasts are liable to take more interest in weed-killer wagons than the weed-killers, although without the latter there would not be the former. Weed-killing falls naturally within the ambit of railway permanent way departments because it preserves the track and replaces hand weeding previously undertaken by plate-layers. The Southern Railway started experimenting with weed-killer in 1931. It used sodium chlorate and Occysol. They changed from pure sodium chlorate to a mixture of sodium chlorate and calcium chloride sold under the name of Altacide. This was less highly inflammable than simple sodium chlorate. The latter got soaked into the PW mens' clothing with obvious implications. The GWR's first weed-killer train did not emerge until 1937 although the company had begun experimenting with weed-killers in 1932. (Parker, 2016)
The Chipman Chemical Company became involved in 1935 and supplied the Southern with spraying equipment as well as chemicals.
The FR definitely had a weed-killer wagon by 1934 so in this respect, as among so many others, it was in the vanguard of progress. (see: Weed-Killing Waggons)
In 1937 the UK's standard gauge railways treated the following length of branch lines (Viner 1938):
LTPB 100 miles GWR 680 miles LNER 800 miles in the Southern Area SR 1,100 miles LMS 1,260 miles
Weed-killer trains seem to have fallen out of use on the standard gauge during WW2 but resumed soon after. Fisons entered the market in competition to Chipman in early 1960 and had a train designed to apply Weedex, an insoluble weed-killer, held in suspension. (Dow 2014)
With growing environmental awareness many chemicals have been banned and the choice of chemical has been affected by the need to reduce environmental impact. By design, railway trackbeds are very well drained to give them the right mechanical properties but this means that soluble chemicals soon pass into the drainage water. British Rail used weed-killer supplied by the agrochemical division of Ciba-Geigy. In 1991 British Rail moved away from using the triazine group of herbicides such as atrazine and started using non-residual glyphosyte. (Parker, 2016) This change was due to the levels of triazine herbicides found in drinking water supplies. Residual chemicals like triazines are effective as weed-killers because they stick to the clay in soil and have a long lasting effect. With foliar applied herbicides, such as glyphosate, which rely on being applied to green leaves, it may be necessary to spray more than once per growing season to get the required degree of vegetation control. Glyphosate is also a translocated herbicide which means that it moves around inside the plant once absorbed. Hence it is effective in killing weeds with strong tap roots etc.
The following quote from Fred Howes summarises herbicide use on the FR (Howes 2016):
"I have no idea what the pre-1946 railway used, maybe Sodium Chlorate – I don’t recall any conversation with Will Jones on that subject. When I first joined the FRCo in 1964 Norman Gurley was the Chief Crop Sprayer, using the rapidly collapsing bogie 67; I used to drive the train and he sprayed everything including me and his lunch. From mid ‘70s I managed the process and many different chemicals and methods were used, including several contractors."
However Cooke's Explosives had a small works at Cwm Nantcol where they manufactured sodium chlorate, as well as the works at Penrhyn. In the 1950s they very generously donated this weed-killer to the FR.  The first report of weed killer use after the FR revival is in 1957 when it was reported in FRSL Newsletter No. 11 that "The whole line effectively treated with weedkiller".
A deep layer of granite ballast is a fairly effective deterrent for weeds so once new granite ballast began to be used, from the mid 1960s onwards, it had a beneficial side effect in respect of weed control. The Hamburg Ballast used by the old FR contained not just river gravel but also finer shingle and sand. It made a better growing medium for weeds. As evidence of this fact, here is a quote from the report of Major E Drewitt on a derailment near Coed-y-Bleiddiau in October 1901 - "The ballast consists of river gravel and shingle, but a good deal of earthy matter is contained in it." (See Day Log/1901-10-04) Little of this Hamburg Ballast was used once ships stopped calling at Porthmadog so the utilisation of weedkiller from the 1930s was timely.
More information will lurk in the F&WHR records.
- Davies V D in Great Railway Eras, Festiniog: The Pioneers' Stories (2007) Davies M and Mitchell V, page 45, Middleton Press, Midhurst, West Sussex, GU29 9AZ.
Dow A (2014) The Railway, British Track Since 1804, Pen & Sword Books Ltd. South Yorkshire, S70 2AS, UK, pp 410-412.
Howes F (2016) Email dated 23/3/16.
Parker T (2016) NRM Search Engine Assistant, email dated 8/4/2016.
Viner Brady (1938) Weed Killing on Railways, PWI Journal, August (quoted in Dow A)