I think the 18 gauge wire will be fine. 18 gauge wire is rated at up to 16 amps at several feet, so with your run lengths likely it could handle 10 amps conservatively. Given you have blocks, you likely would blow the breaker (assuming you have a modern breaker) before the wire melted, especially given the number of feeders you are putting in.
The best test is under load, honestly taking an engine you know draws a lot (like an old Pullmor equipped engine) pulling a lighted car might show you if you have drops and is a better test IMO than a meter.
