Caboose laws was Prototype discoveries
There is an extensive discussion of the processes involved, but by and large, it was by state legislation, usually with respect to labor law. There is a discussion of the process of creating these laws in many states in the new PRRT&HS Cabin Car book, by Bob Johnson. As seems typical, there was definitely a lack of uniformity. Some laws specified length, some the number of wheels, most steel underframes, etc... Some allowed the continued use of bobber cabooses for local/yard work. There was some negotiation between the unions, the railroads, and the states. The Pa bill was enacted in 1913. You are correct that the proliferation of eith wheel cabooses coincided with these laws.
On the PRR, they had just finished building the ND series cabins, which were bobbers with steel underframes. In Virginia, the laws required two four-wheel trucks, but had no lenght requirement, so the PRR added trucks to the ND to make class NDA, which not at all surprisingly, were assigned to the Delmarva (and especially Va) area. The first new all steel N5 was built in 1914, to comply with the laws in all of the states served by the PRR.
From: main@RealSTMFC.groups.io <main@RealSTMFC.groups.io> on behalf of Steve Summers via groups.io <summers1218@...>
Sent: Saturday, September 11, 2021 9:07 PM
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Subject: [EXT] Re: [RealSTMFC] Prototype discoveries
As many older railroads had four wheel cabooses in the past, it would be of interest to have dates that they were no longer allowed. Guessing the proposed outlaw date rules corresponded with a rash of then-new caboose builds.