Votage between ends of transmission line
There's rarely any real reason to need an answer to this question. Presumably B and D are far apart so there's no way to measure the voltage between them, and really no reason why the voltage between them will matter to the operation of the circuits attached at the B end or the D end of the line. 

Obviously if the circuit at the D end is itself grounded somewhere, the design will need to somehow isolate the connection to the transmission line to avoid ground loops (i.e. undesired currents flowing due to ground potential differences between the two ends of the transmission line).

If you really do want to know, Then you can treat the B-D wire an antenna, with the A-C wire as a parasitic antenna element. And the voltage difference between D and B depends what signals are impinging on it from all outside sources.