A railway company is planning the infrastructure for the city and is considering two types of transportation, trains and trams. Each train can transport 120 people per hour comfortably and each tram can transport 30 people per hour comfortably. Since trains take longer to build, the number of trams must be at least twice the number of trains. If the railway company wants to transport at least 600 people per hour, minimize the total number of transportation units required.