treni

One root, no seam. Tren- looks temptingly like tre (very) with something bolted on, and it is not: there is no suffix -n- in Esperanto, and treni is a single indivisible root. Take the ending off and you get tren-, which means nothing on its own and never has.

Drag, not pull. Tiri is the general verb for pulling: you tiras a rope, a door, a tooth. Treni adds friction and effort. You drag something along a surface, hauling it because it is too heavy to lift, and the thing being dragged is usually resisting. A tug pulls a barge with tiri if you are being neutral about it and treni if you want the reader to feel the weight. The reflexive sin treni is to trudge, to drag oneself along, and it is the natural verb for a wounded man crossing a field.

A family you already half know. The trajno you catch is a separate borrowing into Esperanto, from English, but historically it is the same Latin trahere behind both: a train is a thing dragged. So are English trail, tractor and traction. Esperanto keeps them apart as roots - tren-, trajn-, trakt- - which is normal for international vocabulary and is why the etymology helps your memory without giving you a rule.

Around the root. Trenilo is a drag or a sledge, treniĝi is to drag on in the sense of a meeting that will not end, and trenaĵo is the trailing part of something, including the train of a dress.