Railway track is littered with the remains of items stolen from passing freight trains, in Los Angeles. California. File / Reuters

Dozens of freight cars are broken into every day on Los Angeles's railways by thieves who take advantage of the trains' stops to loot packages bought online, leaving thousands of gutted boxes and products that will never reach their destinations.

According to the tags found on Friday by a media team on a track near the city centre - which was easily accessible from nearby streets - many major US mail order and courier companies such as Amazon, Target, UPS and FedEx are being hit by the thefts, which have exploded in recent months.

The thieves wait until the long freight trains are immobilized on the tracks, and then climb onto the freight containers, whose locks they easily break with the help of bolt cutters.

They then help themselves to parcels, ditching any products that are difficult to move or re-sell, or are too cheap, such as COVID-19 test kits, furniture or medications.

Rail operator Union Pacific (UP) has seen a 160 per cent rise in the thefts in Los Angeles county since December 2020.

Police and security agents have arrested more than 100 people in the last three months of 2021 for "trespassing and vandalising" Union Pacific trains.

"While criminals are being caught and arrested, charges are reduced to a misdemeanor or petty offence, and the person is back on the streets in less than 24 hours after paying a nominal fine," said a spokesman for the rail operator.

"In fact, criminals boast to our officers that there is no consequence," he said.

Union Pacific wrote to the Los Angeles County attorney's office at the end of December asking them to reconsider a leniency policy introduced at the end of 2020 for such offences.

The operator estimates that damages from such thefts in 2021 amounted to some $5 million, adding that the amount in claims and losses "does not include respective losses to our impacted customers" or the impact on Union Pacific's operations and the entire Los Angeles County supply chain.

Agence France-Presse