I'm uncertain where Fort Polk is, however being in the industry, the article raises an eyebrow or two.
Creosote ties last 30-40 years or longer. This is depending on location, of course, though additives or cross treatments with Borate to prevent Formosan Beadle attacks have lengthened life cycle times in the Southeast, where wood ties have historically survived the least time.
Secondly, it takes a lot of recycled one gallon milk jugs and energy to make a plastic tie, therefore they are up to 3 times more expensive than a treated wood tie.
Knowing that these are being installed on a Government facility though, helps to explain why the extra money is being spent.