Left silk radical — the indexing component. 线 belongs to the thread/cord family: 绳 (rope), 纸 (paper, originally pulp matted from fibers), 细 (fine), 织 (weave). The radical signals anything filament-like, from sewing thread to a subway line.
Right 戋 supplies the sound (jiān → xiàn, j/x labial shift) and a faint flavor of slender/thin — fitting for a thin thread or wire. Same phonetic family as 钱 (money, originally small coin), 浅 (shallow), 笺 (note).
