List a custom field from an entire post type

function tcp_list_timetable_route_ids( $args = array() ) {
	if ( ! post_type_exists('route') ) {
		// Fail silently
		return;
	}
	$defaults = array(
		'before'		=> '<div class="tcp_route_list">',
		'after'			=> '</div>',
		'sep'			=> ' ',
		'use_color'		=> false, // TODO - doesn't work
		'show_circle'	=> false,
		'route_name'	=> 'long_name',
		'show_alert'	=> false, // TODO - doesn't work
		'alert_markup' => 'default',

	);
	$args = wp_parse_args( $args, $defaults );
	// Returns a sorted array of timetable objects.
	function tcp_get_timetables() {
		if ( ! post_type_exists( 'timetable') ) {
			// Fail silently
			return;
		}
		// timetable sort
		$timetable_args = array(
			'post_type'	  => 'timetable',
			'numberposts' => -1,
			'orderby'	  => 'ASC',
		);
		return get_posts( $timetable_args );
	}

	$timetable_posts = tcp_get_timetables();
	$timetables = array();

	// Format and output each timetable in the database
	foreach ( $timetable_posts as $timetable ) {
		// Add formatted timetable link to an array
		$timetables[] = '<a href="' . get_the_permalink($timetable->ID) . '" class="' . $timetable->post_name . '">' . $timetable->route_id . '</a>';
	}

	echo $args['before'] . join( $args['sep'], $timetables ) . $args['after'];

}

tcp_list_timetable_route_ids();