Ethernet connection using only one twisted pair
Is it possible to create an ethernet bus line using only one twisted pair?


Absolutely Yes. It won't be a bus as 10basex is, but will be point to point as most Ethernet implementations are today. 

Read up on IEEE Std 802.3bw-2015 100BASE-T1 –Automotive Environment
This is a 100Mbps link over a single twisted pair. 

You can also get out to 1000Mbps using IEEE Std 802.3bp-2016 1000BASE-T1 –Automotive and Industrial. Good for router powered ethernet based sensors.  

This might be a good place to start, and it provides pointer to the newer SPE standards allowing both signal and POE over a single twisted pair.