Perhaps you thought that because ein doesn't have an ending for neuter nominative?
Adjectives don't inflect exactly like ein, though.
The endings they take depend on whether there's something in front of them, and if so, what.
After a definite article (der, die, das etc.), they take weak inflection -- this is -e in the nominative singular for all genders, and also for the accusative singular for neuter and feminine (where nominative = accusative always). All other combinations are -en in weak inflection (i.e. masculine accusative singular; all cases in the plural; and genitive and dative of all genders).
Ist the declension for neuter in nominative -es?
No, the weak inflection is -e in the nominative singular (for all genders).
Thus das rote Fahrrad.
The -s on das already shows neuter nominative, so rot doesn't have to do so as well.
On the other hand, "a red bicycle" would be ein rotes Fahrrad -- since ein has no -s ending, rot has to have one.