src/main/java/com/conveyal/gtfs/loader/ReferenceTracker.java
public Set<NewGTFSError> checkReferencesAndUniqueness(String keyValue, int lineNumber,
        Field field, String value, Table table) {
        return checkReferencesAndUniqueness(keyValue, lineNumber, field, value, table,
            table.getKeyFieldName(), table.getOrderFieldName());
    }
public Set<NewGTFSError> checkReferencesAndUniqueness(String keyValue, int lineNumber,
        Field field, String value, Table table) {
        return checkReferencesAndUniqueness(keyValue, lineNumber, field, value, table,
            table.getKeyFieldName(), table.getOrderFieldName());
    }
During table load, checks the uniqueness of the entity ID and that references are valid.
NOTE: This method defaults the key field and order field names to this table's values.

@param keyValue   key value for the record being checked
@param lineNumber line number of the record being checked
@param field      field currently being checked
@param value      value that corresponds to field
@param table      table currently being checked
@return any duplicate or bad reference errors.