It’s common to bundle multiple construction projects into one, either to save money or to take advantage of a charismatic piece of infrastructure that can fund the rest. For example, on-street light rail is frequently bundled with street reconstruction or drainage work, and rail lines can also be bundled with freeway construction in the same corridor (as in Denver) or widening the road they run under (as in New York). Combining different constructions into one project can be a powerful cost saver, as seen in the Denver example and also in Houston.
The problem is when it leads to scope creep. In case there is one charismatic project that carries the rest, it’s always tempting to add more features to the project to get more funding. If the funding comes from a pot specific to one use – in the examples in this post transit, but it could be anything – then it will also lead to a misleading reporting of the total cost, making it look higher than it is. Part of the surreptitious underfunding of transit in the US comes from such bundling, for example parking garages for commuter rail. More commonly the projects in question will be transit, just not necessarily cost-effective on their own.
Because one agency tends to have the lead on such projects, there is no incentive for cost control. The worst case I know of is high-speed rail construction on the Caltrain corridor; the segment from San Francisco to San Jose incurred the highest cost overrun in the system, its cost rising by a factor of nearly 3 versus a systemwide average of 2, and most of the overrun came from tunnels and viaducts reinforcing various agency turf boundaries.
The flip side is bundling projects not so that a charismatic major project can support others, but rather so that a major project can get the support of others by throwing them bones. This is essentially Amtrak’s Vision plan for the Northeast: Gateway is meant to get support from New York and New Jersey now that ARC is canceled, Market East is meant to get support from Philadelphia on the dubious idea that the city wants a Center City stop, and so on. In this case, there is a symbiotic relationship: the charismatic project, in this case HSR, gets to brand all these separate projects as necessary for a grand goal, while the presence of the smaller project ensures that local politicians, whose priorities rarely include providing intercity transportation maximally efficiently, support the project.