One option is to get taxis at the train station in Colón. You'll need at least two -- even if one is a minivan. And that will be expensive. The other option is to make arrangements with a transportation company to meet you at the station. There are a number of transport companies in Panama. One well-known company is Roadrunner. You'll also probably be contacted by a TA poster who pushes the envelope of acceptable posts by promoting her tour company under the guize of offering local advice if you'll send her a private message. Buyer beware.
When i took my Boy Scouts to Panama a few years ago, we hired Edwards Transport. Since it was a slightly larger group, we hired a small touring bus. The driver met us at the Colón train station and from there we went to Gatun Locks, Fort San Lorenzo and Portobelo, where we stayed overnight. I was very impressed with Edwards Transport. They are among the few who are fully insured, their equipment was in very good shape and the driver was safe, knowledgeable and very customer focused. If you're interested, you can reach them through their website www.edwardstransport.com or by phone at 011-507-261-9314. (If you are in Panama, you only need to dial the last seven numbers.)
I'm curious -- why do you want to go to Isla Grande, too? That's going to make it a long day in a van.