It is too soon to tell what caused the Amtrak train crash that killed seven people on May 12. But advocates of increased government spending are already beginning to use the crash to promote more spending on infrastructure and are criticizing Republicans who voted to reduce Amtrak’s budget the day after the crash.

Yet there is a flaw in the assumption that spending more money would result in better infrastructure. In fact, in some cases, the problem is that too much money is being spent infrastructure, but in the wrong places.

The reason for this is that politicians prefer to spend money building new infrastructure over maintaining the old. The result is that existing infrastructure that depends on tax dollars steadily declines while any new funds raised for infrastructure tend to go to new projects.

We can see this in the Boston, Washington, and other rail transit systems. Boston’s system is $9 billion in debt, has a $3 billion maintenance backlog, and needs to spend nearly $700 million a year just to keep the backlog from growing. Yet has only budgeted $100 million for maintenance this year, and instead of repairing the existing system, Boston is spending $2 billion extending one of its light-rail lines.

Similarly, Washington’s Metro rail system has a $10 billion maintenance backlog, and poor maintenance was the cause of the 2009 wreck that killed nine people. Yet, rather than rehabilitate their portions of the system, Northern Virginia is spending $6.8 billion building a new rail line to Dulles Airport; D.C. wants to spend $1 billion on new streetcar lines; and Maryland is considering building a $2.5 billion light-rail line in D.C. suburbs.

On the other hand, infrastructure that is funded out of user fees is generally in good shape. Despite tales of crumbling bridges, the 2007 Minnesota bridge collapse was due to a construction flaw and the 2013 Washington state bridge collapse was due to an oversized truck; lack of maintenance had nothing to do with either failure.

Department of Transportation numbers show that the number of bridges considered structurally deficient has fallen by more than 50 percent since 1990, while the average roughness of highway pavement has decreased. State highways and bridges, which are almost entirely funded out of user fees, tend to be in the best condition while local highways and bridges, which depend more on tax dollars, tend to be the ones with the most serious problems.

Before 1970, almost all of our transportation infrastructure was funded out of user fees and the United States had the best transportation system in the world. Since then, funding decisions have increasingly been made by politicians who are more interested in getting their pictures taken cutting ribbons than in making sure our transportation systems run safely and smoothly.

Proponents of higher gas taxes and other increased funding on infrastructure may talk about crumbling bridges, but what they really want is to spend more money on new projects that are often of little value. For example, they want high-speed trains that cost more but go less than half the speed of flying and light-rail trains that cost more but can move fewer people than buses.

This country doesn’t need more infrastructure that it can’t afford to maintain. Instead, it needs a more reliable system of transport funding, and that means one based on user fees and not tax subsidies.