These are spaced about every 300 meters next to the trail in Memorial Park in Houston, TX, but they aren't only along the trail. They are throughout the park. Does anyone have an idea what they might be? Thanks!
These are small cell antennas, generally used for mmWave 5G due to the poor penetration the high frequency signals have or to increase capacity of a network where a lot of devices are used in a small space (places like stadiums, airports, city centers, etc) in which case they may have LTE antennas as well. They usually cover about a city blocks worth of area so you’ll see them spaced much like you describe.
The box on the ground is the actual radio and power supplies with the antennas behind the shroud on the pole. You’ll sometimes also see the radio cabinet mounted about 1.5m up the pole so it’s off the ground.
Source: a company I used to work for installed these (Crown Castle).