The missing 50% either did not earn their keep or lost money for the railroads when the railroads were forced by Government fiat to keep them.
After deregulation,those lines that were marginalable to be sold to Short Lines that are able to operate them for less, thus the Short Lines are able to make money.
What lines are left, and being operated by the Class Is are being improved.
BNSF is double-tracking the last remaining segment of their Southern Route thru New Mexico. When completed, it will more than double the capacity of the line.
Union Pacific has been adding more tracks to it route thru Nebraska and Kansas, increasing it's capacity.
Without the government regulation of route and rates, the railroads are able to price their services as the business allows, rather than go thru a government hearing that took sometimes years.
All four major roads, BNSF, UP CSX and NS are upgrading their facilities to handle an ever increasing traffic load.
I worked for the Milwaukee Road back in the late 70’s early 80’s. they were coming out of bankruptcy. Tracks were limited to 10mph in many areas. We worked hard to bring the track up to a 30 mph speed. After 3 years it was up to 50. They spent millions of dollars upgrading. For a time they were merged with the GTR (Grand Trunk) and now I have no idea who owns the company. But they had a key line from KC to Chicago that survived over a parallel Rock Island line that was just 20 miles west but ended up in the same place side by side.
The trains roll through that area now where I used to live on first class tracks moving a lot of freight. We took pride in bringing back the Milwaukee Line from bankruptcy to preserve the vital line. Sadly, some sections of track did not survive and many small towns lost the rail spur to the local grainery. Trucking was cheaper at the time. But now trucks are not nearly as economical. But the small towns without rail access are getting hit because trucks are the only option they have. It is unfortunate but that is economics as it was 30 years ago compared to now.
Deregulation has been good. But the government put up hundreds of millions of dollars propping up many railroads. Maybe the payback to the economy will return to us?