Seconded to the federal government in World War II, he played a pivotal role in the manufacture of fighter aircraft in the defence of Australia.

He was then engaged by the Interborough Rapid Transit Co., and among other work was in charge of electrification of the West Jersey and Seashore division of the Pennsylvania Railroad.

Although his approach to industrial relations has been judged as paternalistic, his concern for workers' conditions was genuine and he was personally responsible for improvements such as better sanitation facilities and the provision of decent cafeterias at Newport Workshops.

Improvements in service reliability earned the ultimate compliment by way of the following complaint published in a Melbourne newspaper: "Mr Clapp's fiendish efficiency means that we have lost another excuse for being late for work in the mornings.

He shared his awareness of people making use of his surname's unfortunate association with "the clap", a sexually transmitted infection: when addressing a group of railwaymen he said, "I know what you fellows call me behind my back.

This reflected not only a desire to boost rail traffic through increased farm production and customer demand, but also a belief that the role and social responsibilities of the Victorian Railways as an organisation went far greater than simply the provision of transport.

In other key developments that drew on his US railroad experience, he oversaw the introduction of electric lighting on VR locomotives and the fitting of auto-couplers.

[10] In 1935 Clapp raised the maximum line speed limit on Victorian Railways from 60 to 70 mph (96 to 112 km/h)[11] and the Flier subsequently ran to Geelong on a 55-minute timetable.

[13] Similarly, the Adelaide Express was renamed The Overland in 1926, despite some reluctance from the South Australian authorities who jointly ran the inter-capital service.

Clapp oversaw the reduction in running time for the service from sixteen and a half hours in 1928[14] to around fourteen hours in 1938 (with the departure time changed from 4:30pm to a much more convenient 7:00pm),[1]: 157  achieved at very moderate cost simply through the introduction of Automatic Staff Exchange equipment between Ballarat and Serviceton to allow unimpeded progress from one track section to the next, and design improvements to the existing A2 class locomotive fleet to increase power and efficiency.

On his return, he began work on his ultimate project, upgrade of the Sydney Limited service into the all-steel, all-air-conditioned, non-stop, high-speed express streamliner, Spirit of Progress.

The upgrade of the service was then a smart investment in promoting the Victorian Railways brand and its reputation to the nation's most powerful and influential people.

[2] Although Beaufort bombers were advanced aluminium aircraft requiring complex and previously untried assembly processes, the first flew as early as August 1941.

[15] Clapp was appointed a Knight Commander of the Order of the British Empire (KBE) in the 1941 New Year Honours "[f]or public services in the Commonwealth of Australia.

New South Wales refused to ratify the agreement as only a relatively small proportion of the project cost would be spent in NSW, where almost all track was already standard gauge.

On 14 July 1952 the first of the B class diesel electric locomotives, which were to revolutionise train operations in Victoria, was delivered to the VR and travelled from NSW to Melbourne's Spencer Street station under its own power.