---Updated July 25, 2022, by Kimberly Walters---
Upgrading infrastructure in the United States of America has long been delayed and is now reaching a critical point. Replacing railroad bridges that are old or have exceeded their useful design life is just as important as any other element of our nation’s infrastructure. Bridges are a small component of an extensive railroad system. However, if taken out of service, a whole line could be shut down. Railroad companies would lose time, expenses would increase, and commodities would be delayed.
Rail Transportation in America
According to the Federal Railroad Administration (FRA), the U.S. freight rail network is widely considered the largest, safest, and most cost-efficient freight system in the world.  The nearly $80-billion freight rail industry is operated by seven Class I railroads  (railroads with operating revenues of $490 million or more)  and 22 regional and 584 local/short line railroads.  It provides more than 167,000 jobs  across the United States and offers ancillary benefits that other modes of transportation cannot, including reductions in road congestion, highway fatalities, fuel consumption, greenhouse gases, cost of logistics, and public infrastructure maintenance costs.
The employment of railroads for freight transportation is second to truck transportation in terms of usage. Railroads and pipelines are two modes of private mass transportation, moving bulk commodities. The railroad industry would have to continuously invest in its infrastructure to allow them to stay competitive with other transportation methods.
Why Should You Replace a Railroad Bridge?
As train technology evolves, they become both heavier and faster. Because of this, older bridges, often made with timber, have approached the end of their efficient design lifespan. Companies must replace these older bridges if they want them to support contemporary trains. Advanced trains need newer structures made of steel and concrete that not only add capacity to the rail line but also allow trains to cross them at higher speeds, therefore increasing train efficiencies.
The Processes of Replacing a Railroad Bridge
The standard method of replacing a bridge consists of three fundamental steps.
Step 1: Install a new substructure
This begins by driving pilings and building new piers. The work crew starts under the bridge while it remains open to train traffic. Contractors receive short windows of time to drive the pilings through the existing bridge and construct abutments and piers underneath. When a train approaches, the workmen must remove all the equipment so as not to obstruct the track or delay the train.
Step 2: Remove and replace the superstructure
Companies must schedule a block of time for this process which necessitates a temporary shutdown. This scheduling needs to be done a month or more in advance. During this endeavor, crews must work 24-hour days to replace the old deck and piers with the new spans. The length of time to shut down traffic is a function of the volume of the traffic the bridge receives and/or the size of the bridge. The time frames needed for the switch out have varied from 4 to 16 hours with an average of 8 hours. Exceeding the time window for the work is not an option as delaying one train can affect the whole line.
Step 3: Finish any remaining work
After the spans are switched out, work crews will install any remaining backfill around the new abutments and ballast the approaches using a hi-rail dump truck. Then they align the new track with the existing railway and replace any damaged ties.
Other Ways to Replace a Railroad Bridge
Another method of replacing railroad bridges is employing cranes for the task. In this case, a sizable crane allows new bridge spans to be delivered and decked on the ground. This has resulted in improving both speed and safety. Crews then pick up the old span and place it on the ground so that it can be dismantled in a safer location.
Employing a mobile gantry system for span replacements is a technology that will assist Class I railroads in completing the project more quickly. The gantry lifts, removes, and installs spans from above the bridge without the need for a crane and crane pad. This benefits both efficiency and the environment.
Accelerated Bridge Construction (ABC) is another mode for railroad bridge replacement. According to a 2016 report from the American Society of Civil Engineers, almost 40 percent of the nation’s bridges have been in existence for 50 years or longer. Also, 9.1 percent of bridges are structurally deficient. ABC reduces traffic disruptions and risks for both drivers and construction workers since it shortens the time it takes to install a bridge over a railway. The process involves constructing large portions of bridges offsite, then installing them quickly, often within 48 to 72 hours. Doing so can reduce road closures, traffic delays, and overall project costs.
Other benefits include increased worker safety, reduced environmental impact, and improved bridge quality and longevity. One risk in using ABC is that it relies on specialized equipment, like self-propelled modular transporters (SPMTs), to move the prefabricated elements into place. The workers must properly manage this equipment at the worksite and consider all load cases for the bridge for the installation.
Additional Bridge Replacement Considerations
Railroad companies need to factor in the weather when selecting times to replace bridges. For example, a bridge replacement conducted in April after the river freezes must take into account the amount of ice flowing down the river. In this situation, it would be necessary to pull the causeway out of the river to allow the ice to pass. Then it is built out again to continue working on the permanent bridge.
There are also bridges built for special purposes that can be looked at. These include 1) Alaska Railroad Corp. building a bridge that will improve U.S. military access to important training facilities; 2) Norfolk Southern Railway’s modifying two bridges to accommodate wide and tall equipment for two utility plants; 3) CSX Transportation replacing bridges to solicit more business in certain corridors.
One topic worthy of further study includes increasing the knowledge of long-span railroad bridge design. Other topics are improving the maintenance of existing deteriorating bridge approaches and examining longitudinal loads (their magnitude and distribution) in railroad bridges.
 See Railway Technology, “The world’s 10 longest railway networks,” February 2014; Association of American Railroads, “Overview of America’s Freight Railroads,” March 2020.
 The seven Class I freight railroads are: BNSF Railway Co., Canadian National Railway (Grand Trunk Corporation), Canadian Pacific (Soo Line Corporation), CSX Transportation, Kansas City Southern Railway Co., Norfolk Southern Combined Railroad Subsidiaries, and Union Pacific Railroad Co. The combined revenue of Class I railroads equaled $67 billion in 2017.
 Association of American Railroads, “Overview of America’s Freight Railroads,” March 2020.
 Association of American Railroads, Railroad Facts, 2019 Edition, 3.
 Railroad Facts, 2019 Edition, 3.
About J.F. Brennan Company
J.F. Brennan Company, Inc. (Brennan) is a fourth-generation, family-owned, marine construction firm that specializes in environmental remediation, dam construction, commercial dive, harbor management, and submarine cable services. Working closely with public and private owners of water-based infrastructure since 1919, Brennan operates throughout coastal and inland waterways nationwide, maintaining a large fleet of marine equipment backed by more than 600 maritime professionals.