The work as part of the Great North Rail Project involves Coppull Moor Lane and Chorley Lane railway bridges.
Both are being grit blasted to expose the metal structures so vital repairs and repainting can take place.
Brickwork will also be maintained on the bridge supports, which are known as abutments.
The important repairs have started on both bridges and are due to be finished in July.
Engineers will carry out the refurbishment work during the day between 7:30am and 5:30pm, and overnight on Saturdays between 10:30pm and 8:30am on Sunday mornings.
During the Easter and early May bank holidays, work will ramp up taking place 24 hours a day from:
- 11:15pm on Saturday 20 April until 11:15pm on Easter Monday (22 April)
- 11:15pm on Saturday 4 May until 11:15pm on May Day bank holiday (6 May)
Sarah Padmore, scheme project manager for Network Rail, said: “As part of the Great North Rail Project, we are carrying out vital repair work to two road bridges in Chorley to ensure they remain safe and viable for years to come.
“To assess and repair the bridge structures, we need to grit-blast them. Both bridges will then be repainted. We have written to residents to let them know about our work.”
“We thank the local community and road users in advance for their patience while we deliver these essential railway bridge upgrades.”
Traffic lights will be in place on Chorley Lane bridge so the work can be safely carried out.
Access for pedestrians and cyclists will be unaffected during the projects.