Function build_train 

pub fn build_train(
    repo: &Path,
    base: &str,
    prs: &[(u64, String)],
) -> Result<Train, String>

Builds a speculative train in repo (a non-bare clone whose worktree this function owns): detaches at base, then merges each (id, head) in order with --no-ff. A conflicting PR is excluded (merge aborted) and the train continues without it.

Errors

Git failures other than merge conflicts (missing oids, dirty repo).