According to the proposal prepared by The Boring Company for the City of Kyle, the pedestrian underpass will be constructed to cross the UPRR tracks south of Kyle and "will accommodate multimodal transportation options both slow and fast moving, including electric vehicles, bicycles, and pedestrians".

The proposal says " TBC has standardized a 12-foot inner diameter and a 13.5-foot outer
diameter tunnel with 9-inch-thick reinforced concrete segments installed by the TBM immediately after
excavation. Once mining of the tunnel is complete, a flat paved road surface is installed near the invert
with a 10 ft width".

For the preliminary engineering study, TBC asks for $US 50.000. The study consists of a site visit, coordinating the utility & existing structure, geotechnical investigation,Phase 1 Environmental Site Assessment (ESA), identification of Horizontal and Vertical Alignment Development, Union Pacific Railroad Permitting process, Preliminary Engineering Drawings, preparing Preliminary Project Delivery Schedules and preparation of Design-Build Construction Proposal.

Although not in the proposal, the tunneling (with a TBM) itself is expected to cost in the lower ends of single digit millions of dollars.

The TBC proposal can be downloaded from the link below.

https://kyle.novusagenda.com/agendapublic/AttachmentViewer.ashx?AttachmentID=25255&ItemID=16748

image and some text: The Boring Company Kyle Pedestrian Underpass (from: Railroad Pedestrian Underpass Preliminary Engineering Proposal prepared by The Boring Company)