src/main/java/com/conveyal/gtfs/loader/Table.java
public String getOrderFieldName () {
        String name = fields[1].name;
        if (name.contains("_sequence") || compoundKey) return name;
        else return null;
    }
public String getOrderFieldName () {
        String name = fields[1].name;
        if (name.contains("_sequence") || compoundKey) return name;
        else return null;
    }
Returns field name that defines order for grouped entities or that defines the compound key field (e.g.,
transfers#to_stop_id). WARNING: this field must be in the 1st position (base zero) of the fields array; hence,
this MUST be called on a spec table (i.e., one of the constant tables defined in this class). Otherwise, it could
return null even if the table has an order field defined.