The source says "they drank to the Engineer, and the Contractors, and the Bankers who had lent them the money, and to the success of the other railways springing up around them, including the Mid-Wales, the first sod of which was to be cut in a few days' time" [i]. The Mid-Wales Railway was incorporated by an Act of 1 August 1859 to build a railway from Llanidloes to Llandovery. [ii] It was common practice for navvies to follow work around the country. The song goes on to say "When these few days are over, / The navvies they will part, / And go back to their gangers"

References:
[i] Gasguoine, C. P. The Story of the Cambrian: A Biography of a Railway.: (Wrexham and Oswestry, Woodall, Minshall, Thomas & Co, 1922) pp. 23-24
[ii] https://spellerweb.net/rhindex/UKRH/GreatWestern/Narrowgauge/MidWales.html (accessed 06Feb21)