Railroads (and other "right-of-way" companies) could use their lines as they saw fit, including imterconnecting with Bell lines at their PBX (including dial PBXs). This was true whether the company owned the lines or leased them from Bell.
The rules were indeed different on private lines. Generally the customer could hang anything they wanted on leased lines as long as they did not cause interference outside the bandwith. (Leased lines included telegraph, teletypewriter, voice, program channels [audio channels with wider bandwith than voice-grade channels], television channels, and various grades of data channels.)
Most independent companies were even more restrictive than Bell in their regulations, both for lines attached to the switched network and for leased lines.
Wes Leatherock snipped-for-privacy@aol.com