In 1843, he took over the running of a family business originally called Neilson & Mitchell, then Neilson & Co.[4] The company was based in Hydepark Street in central Glasgow before expanding to include nearby Finnieston Street. The company started by building ship engines but quickly expanded to building locomotives, at the time a fledgling industry, but with the British empire still expanding, a very lucrative one.[2] Neilson locomotives were exported all over the world and examples still exist in far off places like India, South Africa and New Zealand.[2]
On the death of his father in 1865, Walter inherited the Queenshill property in Kirkcudbright, the property was named after the resting spot of Mary, Queen of Scots, fleeing to safety after the defeat of her army at Langside in 1568.
Neilson had partnered with Burns, Baumgarten & Co. in Glasgow; he retired from this partnership as of 1 January 1873.[5]
The locomotive business flourished and when Walter finally retired in the late 1870s,[2] the name changed again, this time to Neilson, Reid & Co. James Reid had been associated with the company for a long period[3][6] and even though Reid and Neilson had a falling out, the company went on to merge with others into North British Locomotive Co. Ltd.[4] Walter also supplied expertise and advice to the French builders of the Suez Canal.[citation needed]