Whitney's theorem allows isomorphism testing for connected simple graphs, apart from
K3 and
K1,3, to be formulated as distinguishing their line graphs. However, the relation between fixed-dimensional Weisfeiler--Leman (WL) expressivity on line graphs and on their roots remains unresolved. We study this relation through Implicit Line-Graph WL (ILG-
k-WL), which is exactly
k-WL on
L(G), executed over the edges of
G with line-graph relations derived from endpoint incidence and without explicitly constructing
L(G). On the Whitney-general class, the relation between root-domain and line-graph WL depends on
k. For
k=1,2, ILG-
k-WL adds no distinguishing power beyond root-domain
1-WL and misses some pairs that
1-WL separates. For
k=3, we prove the backward containment
L(G)≡3-WLL(H)⇒G≡3-WLH. Strongly regular witness pairs, including the Shrikhande/rook pair, show that ILG-
3-WL is strictly more expressive than
3-WL. The backward containment also extends to disconnected graphs with no isolated vertices when every connected component is Whitney-general. Deterministic ILG-
3-WL separates all three substructure-counting witness pairs, all
105 pairs in SR25, and
359 of
400 BREC pairs. An untrained dense ILG-
3-GNN gives the same pairwise verdicts on these evaluations.