。
Look at the train running over the bridge.
Literal
Train [subject-が] bridge [of-の] top [object-を] is-running [nominalizer-の] [object-を] look [try-てごらん].
~てごらん is a gentle imperative used by an adult to a child, softer than ~なさい or the bare imperative. The nominalizer の + を ('the (thing that is)...') turns 列車が橋の上を走っている into the object of 見る — 'look at the train-running-over-the-bridge.' 橋の上を with を marks the path traversed.