I wouldn't dismiss that kind of infrastructure spending so quickly. Even before shovels are in the ground, there are probably just as many jobs in designing the project as there are in building it. A highway widening, a new rail line, or an urban transit project could have a dozen consulting firms designing the project, ranging from civil engineers to fisheries specialists. There doesn't have to be actual construction for there to be a stimulus.
As an engineer myself and currently supporting a brother with a master's in engineering looking for work, I am acutely aware of the design phase. However, the bulk of employment in any project comes after the design phase. That's reality.
If you want to create jobs, you have to you have to do that by creating construction work. That cannot be done quickly by implementing projects which have long consultation, EA and design phases. The most extreme example of this is the proposal to consider Ontario-Quebec HSR as a stimulus program. With good timing that might turn out to be stimulus for our next recession!
The only solution for this current recession is to tackle accumulated deferred maintenance (ADM). That's not as sexy as high speed or light rail. It's fixing roofs in elementary schools, resurfacing municipal roads, replacing diagnostic equipment at hospitals, rebuilding hangars at military bases, replacing half century old water mains, etc. This is stuff that can be started quickly, does not need EA, often has no jurisdictional conflicts, and will likely result in immediate savings through reduced O&M costs. And you have to keep in mind that the most vulnerable workers are also incidentally construction workers who will have higher unemployment than engineers, consultants, etc. especially as the housing downturn sets in. Better to put that surplus labour to work tackling that 123 billion in infrastructure backlog before addressing the 115 billion we need in new infrastructure.
Lastly, have a look at our most critical infrastructure need. It's water treatment and delivery (averaging well over 30-40 years old) and roads. It's not transit. There is no good having a new light rail line if you don't have safe, clean water and poor roads for the bus to take you to that rail line. As presently proposed the Liberal-NDP stimulus plan appears to be an excuse to deliver new social programs rather than being targeted at solving our current economic malaise.