If there is no coal load, an oil-burner for example, the following may be an option depending how well it blends with other "clutter" atop the tender. The photo (right) shows a mounting on a large HO scale tender.
For all-metal diesel locos or steam locos with no workable tender space, the operator's cab is another option. but hang the antenna vertically so it's not flat against a metal surface. One modeler with a small steam loco replaced the metal cab roof with plastic and placed the antenna flush against the new plastic roof.
Keep in mind however, antenna and radio receiver cannot be separated except by using coax cable. A bit of wire may work but transmission distance will be seriously reduced.
Adapter and radio receiver can be separated, but four connecting wires are required.