Find entries from related section after certain date
You’d first get all the relevant entries of your “trainings” section, or better get just their IDs as that’s way better on performance.
Then on another element query you set the relatedTo parameter to only get entries related to these entry IDs.

{% set trainingsEntriesIds = craft.entries({
    section: 'trainings',
    postDate: '&gt;= ' ~ 'now'|date('c'),
}).ids() %}

{% set entries = craft.entries({
    section: 'locations',
    relatedTo: {
        targetElement: trainingsEntriesIds,
        field: 'entriesFieldHandle'
    },
}).all() %}