28V Stepdown with TracoPower TRN 1-2411 DC/DC converter for ESP8266?
The simple answer is maybe, but with several caveats.


Regulator Output Voltage:


As JRE notes, you must verify that the ESP8266 module you selected accepts 5V. I see that, for example, the SparkFun ESP8266 module (WRL-13678) only operates with an input voltage of 3.0-3.6V. A 5V regulator would be no-go for that module.


Regulator input voltage range:


A 28V battery should be fine with a regulator input range up to 36V. OK there.


Check the supply current requirement for the module you have selected. 


For example, the SparkFun WRL-13678 datasheet says it can draw up to 215 mA. A 200mA regulator would be marginal at best. Conservative design practice dictates use of a regulator able to provide 1.5X the maximum load current. And don't forget to add in the current draw for any additional components you want to use.


Don't forget to use input and output capacitors. 


Most voltage regulator modules REQUIRE external input and output capacitance. Use the voltage regulator data sheet Applications section to determine what external components are necessary.

Finally, do you actually need an isolated output regulator? They are considerably more expensive. The TRN1-2411 is about $10. A simpler and cheaper choice would be something like the CUI P78E05-1000 which provides 5V @1A for about $3. For a 3.3V module try the CUI VXO7803-500 to get 3.3V @500mA for $2.50. I use the helpful search filters on digikey.com to shop for appropriate and inexpensive components.

In summary:


Verify regulator output voltage matches module input voltage range.
Verify the raw power source (battery) voltage is well within the input voltage range of the regulator.
Verify the regulator can supply plenty of current for the selected module.
Follow the regulator's datasheet for external component requirements.