Here in South Australia we had the Bluebird class cars which bore an uncanny resemblance to the Budd railcars, a number have been preserved, they were nice machines. They were still operating when i arrived in 1981 and I rode on them quite a lot in the 1980s when i was doing country service in Mt Gambier. https://en.wikipedia.org/wiki/South_Australian_Railways_Bluebird_railcar
The Boston and Maine was one of the first railroads to purchase Budd RDCs, aka Budd Liners for us who rode them on the B&M. The reason was to point blank save money for the railroad because they served many branch lines and a vast commuter service. They wanted to not only save money but also decrease the turnaround of the trainsets at the end of the branches, and this is where the Budd Liners succeeded.
In the end, they mustered up the largest collection of them in the world when they inherited them from many railroads plus purchased others to keep their own fleet operational. When Conrail gave up running commuter railroads for various operators including Amtrak, the B&M inherited many from that operation. By this time, they were operating under contract for the MBTA, and we saw many, many RDCs from various companies still in their original liveries. On one particular trip, I saw various roads in the yard at North Station. It was amazing and sad feeling too to see such roads as Lehigh Valley, Reading, New Haven, New York Central, Boston and Albany, Baltimore and Ohio, Jersey Central, and so many others. On another trip, I rode in a CPR painted RDC which was pulling others lettered for the Reading, and New Haven.
Eventually, they started to die due to their age and constant use. After the famous Blizzard of '78, they found that the venerable Budd Liners couldn't do it anymore like they used to. The problem is that they needed commuter trains and in the end they kept the Budd Liners as coaches by removing the drivetrain and transmission but kept the diesels to run the A/C units on the roof. By this time, the MBTA had purchased reconditioned and refurbished F8s and F9s from Morris and Knudsen. These were once Gulf Mobile and Ohio units and became FP10s. MBTA painted them in a pink, silver, and gold scheme we called "Easter Eggs". The trains were FP10s pulling or pushing Budd Liners with some still used as cab cars until the "T" got some funding to replace these trainsets with the ubiquitous F40PHs and Horizon cars which we still have today.