src/main/java/com/conveyal/gtfs/loader/Table.java
public List<Field> requiredFields () {
        // Filter out fields not used in editor (i.e., extension fields).
        List<Field> requiredFields = new ArrayList<>();
        for (Field f : fields) if (f.requirement == REQUIRED) requiredFields.add(f);
        return requiredFields;
    }
public List<Field> requiredFields () {
        // Filter out fields not used in editor (i.e., extension fields).
        List<Field> requiredFields = new ArrayList<>();
        for (Field f : fields) if (f.requirement == REQUIRED) requiredFields.add(f);
        return requiredFields;
    }
Get only those fields marked as required in the official GTFS specification for this table.