no need to switch trains..take one all the way from retiro. tren de la costa is a rip, very expensive...plus the hassle of changing trains!
once in tigre, either buy a round trip ticket for $15 or you can reserve lunch, always a pleasant day out. el gato blanco is excellent but expensive, la vaca something or other has become run down and the food not so good...i went camping in tigre a month or so ago, in a place called luz y fuerza: $15 round trip boat ride, $12 entrance fee per person , and $12 per night per person...lots of places to put tents.