File:DTX with RAB alternatives.svg

English: Caltrain Downtown San Francisco Extension (DTX), potential routes (from 2018 Rail Alignment and Benefits study, RAB). The approved DTX alignment takes the line underground near 7th & Berry, follows Townsend to 2nd, and then turns north on 2nd into the Transbay Terminal. RAB proposed two potential realignments:

1. Pennsylvania Avenue Extension (PAX) would take the line underground near the southern portal of Tunnel 2, at 25th, and use a bored tunnel following Pennsylvania to the approved DTX alignment
2. Mission Bay alignment (MB) would diverge from the existing line near the northern portal of Tunnel 2, at 23rd, and use a bored tunnel under 3rd, joining the approved alignment between 3rd and 2nd on Townsend

In addition, RAB proposed loops out of the northern end of Transbay Terminal to facilitate train operations. Four loops were proposed:

1. Main: the tightest loop would follow Main to Embarcadero, then Townsend and would be reserved for Caltrain operations only (not high-speed rail, CAHSR)
2. Spear: an alternative Caltrain-only loop could follow Spear to Embarcadero and Townsend. Both the Main and Spear loops could be routed under AT&T Park if the Mission Bay alignment was chosen.
3. Steuart: the tightest loop that would accommodate both Caltrain and CAHSR
4. Bay Loop: would also accommodate both Caltrain and HSR; this would be built into San Francisco Bay, just outside the limits of the existing piers.

When DTX is complete, a walkway will be built under Beale between the Transbay Terminal and Embarcadero to facilitate transfers to and from BART.