I assume you are referring to HO, but the same happened to Athearn and Roundhouse N.
Horizon Hobbies bought both Roundhouse and Athearn, and continues to make most of both lines. The reissues under the Roundhouse name have been RTR only, no kits. The new Athearn line still has some kits, including some ex-roundhouse items.
Model Power bought Mantua, and are reissuing some of the line under the “Classics” name.
In both cases, some improvements have been made with the re-issues.
Walters bought Life-Like, who had bought most of the Varney line decades ago. Walters has kept the Proto2K portion (nothing to do with Varney) of Life-Like going, but I believe the low end stuff (ex-Varney cars) is history.
Bowser bought Penn-Line, Stewart, some Varney locomotives, Cal-Scale, Cary, and Selly. The locomotives (except for Stewart which were fine as-is) have been improved and are generally available. Bowser also bought some other lines of die cast locomotives (Arbour Models and one other) but probably cannot re-use the dies.
I’m not sure what happened to Silver Streak and Ulrich. I believe some Silver Streak is still being made, but the Ulrich line is dead.
Labelle is still alive under new ownership and is producing with improvements.
Of the plastic freight car lines, I believe Accu-Craft, Red Caboose, and Bowser are all original (new) tooling with their respective companies.
Rivarossi was orginally imported by AHM. AHM eventually was reorganized as IHC, and switched their source to Mehano instead of Rivarossi (not sure of the time sequence of events). Rivarossi was imported by Walters for a while, and then was sold to Hornby. I believe they are still selling under the Rivarossi name.
Others can correct any errors, and will have additional information.
Fred W