OMAHA, Neb. (AP) — BNSF railroad has started taking additional safety measures for crude oil shipments because of four recent high-profile derailments in the U.S. and Canada.

The railroad said Monday that is slowing down crude oil trains to 35 mph in cities with more than 100,000 people and increasing track inspections near waterways.

The Fort Worth, Texas, based railroad also is stepping up efforts to find and repair defective wheels before they can cause derailments.

BNSF spokesman Michael Trevino says these additional safety efforts began in response to the recent derailments. One was earlier this month near Galena, Illinois, and the Mississippi River.

The number of rail accidents remains relatively small compared to total rail traffic, but fiery accidents involving crude oil are increasing because U.S. and Canadian oil production is booming.