diff --git a/packages/ui/src/app.css b/packages/ui/src/app.css
index 481c1bc..21fd2fe 100644
--- a/packages/ui/src/app.css
+++ b/packages/ui/src/app.css
@@ -202,9 +202,17 @@ a { color: var(--accent); text-underline-offset: 2px; }
   border-radius: 8px;
 }
 .rail-top:hover { background: var(--hover); }
+.rail-top:focus-visible { box-shadow: 0 0 0 3px var(--accent-wash); }
 .mark { width: 22px; height: 22px; flex: none; color: var(--accent); }
+.rail-identity { flex: 1; min-width: 0; }
+.space-name, .space-sub { overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
 .space-name { font-weight: 600; font-size: 14px; letter-spacing: -0.005em; }
 .space-sub { font-size: 11.5px; color: var(--ink-2); margin-top: 1px; }
+.rail-switch {
+  flex: none; color: var(--stroke); stroke: currentColor; stroke-width: 1.5;
+  stroke-linecap: round; stroke-linejoin: round;
+}
+.rail-top:hover .rail-switch, .rail-top:focus-visible .rail-switch { color: var(--ink-2); }
 .rail-scroll { overflow-y: auto; padding: 4px 8px 12px; flex: 1; min-height: 0; }
 
 .nav { display: flex; flex-direction: column; gap: 1px; list-style: none; margin: 0; padding: 0; }
diff --git a/packages/ui/src/lib/components/Rail.svelte b/packages/ui/src/lib/components/Rail.svelte
index 88ef4cb..674e79b 100644
--- a/packages/ui/src/lib/components/Rail.svelte
+++ b/packages/ui/src/lib/components/Rail.svelte
@@ -118,12 +118,22 @@
 <nav class="rail" id="rail" tabindex="-1" aria-label="Lists, projects and goals">
   <!-- The space's name is also the way out of it: a tab looks at one space at a time, and switching
        is picking a different URI (there is no account to switch under). -->
-  <button class="rail-top" type="button" onclick={() => (session.picking = true)} title="Switch space">
+  <button
+    class="rail-top"
+    type="button"
+    aria-label="Switch space: {space.index.space.value.name}"
+    aria-haspopup="dialog"
+    aria-expanded={session.picking}
+    onclick={() => (session.picking = true)}
+  >
     <span class="mark"><Glyph name="mark" size={22} /></span>
-    <div style="min-width:0">
+    <div class="rail-identity">
       <div class="space-name">{space.index.space.value.name}</div>
       <div class="space-sub">{humans} member{humans === 1 ? '' : 's'} · {agents} agent{agents === 1 ? '' : 's'}</div>
     </div>
+    <svg class="rail-switch" width="14" height="14" viewBox="0 0 14 14" fill="none" aria-hidden="true">
+      <path d="m3.5 5.25 3.5 3.5 3.5-3.5" />
+    </svg>
   </button>
 
   <div class="rail-scroll">
diff --git a/packages/ui/src/lib/components/SpacePicker.svelte b/packages/ui/src/lib/components/SpacePicker.svelte
index 57dc4fb..18e89bd 100644
--- a/packages/ui/src/lib/components/SpacePicker.svelte
+++ b/packages/ui/src/lib/components/SpacePicker.svelte
@@ -83,7 +83,13 @@
   }
 </script>
 
-<div class="picker" class:overlay>
+<div
+  class="picker"
+  class:overlay
+  role={overlay ? 'dialog' : undefined}
+  aria-label={overlay ? 'Switch space' : undefined}
+  aria-modal={overlay ? 'true' : undefined}
+>
   <div class="picker-head">
     <span class="mark"><Glyph name="mark" size={26} /></span>
     <div style="min-width:0">
diff --git a/packages/ui/test/rail-top.test.mjs b/packages/ui/test/rail-top.test.mjs
new file mode 100644
index 0000000..337fdbd
--- /dev/null
+++ b/packages/ui/test/rail-top.test.mjs
@@ -0,0 +1,30 @@
+import assert from 'node:assert/strict'
+import { readFileSync } from 'node:fs'
+import { describe, it } from 'node:test'
+
+const read = (path) => readFileSync(new URL(`../src/${path}`, import.meta.url), 'utf8')
+
+describe('the space switcher trigger', () => {
+  const rail = read('lib/components/Rail.svelte')
+  const picker = read('lib/components/SpacePicker.svelte')
+  const css = read('app.css')
+
+  it('names its action and discloses the dialog state', () => {
+    assert.match(rail, /aria-label="Switch space: \{space\.index\.space\.value\.name\}"/)
+    assert.match(rail, /aria-haspopup="dialog"/)
+    assert.match(rail, /aria-expanded=\{session\.picking\}/)
+    assert.match(picker, /role=\{overlay \? 'dialog' : undefined\}/)
+    assert.match(picker, /aria-label=\{overlay \? 'Switch space' : undefined\}/)
+  })
+
+  it('keeps a decorative disclosure cue visible beside truncating text', () => {
+    assert.match(rail, /<div class="rail-identity">[\s\S]*?<svg class="rail-switch"[^>]*aria-hidden="true">/)
+    assert.match(css, /\.rail-identity \{ flex: 1; min-width: 0; \}/)
+    assert.match(css, /\.space-name, \.space-sub \{[^}]*text-overflow: ellipsis;[^}]*white-space: nowrap;/)
+    assert.match(css, /\.rail-switch \{[^}]*flex: none;/)
+  })
+
+  it('shows keyboard focus on the full trigger', () => {
+    assert.match(css, /\.rail-top:focus-visible \{[^}]*box-shadow: 0 0 0 3px var\(--accent-wash\);/)
+  })
+})